Smith, who has long defended his decision even long after the dossier got discredited and then revealed as a Hillary Clinton campaign oppo-research artifact, hesitated to embrace "regret" in that specific context. Instead, Smith said, "I'm more ambivalent about it than I used to be." 

However, Smith expressed "regrets" that his reporting tied up the first Donald Trump term in questions about the two theories on the 2016 election that progressives seized as their narrative:

One, it was Facebook. Two it was Russia. And lots of media energy went into chasing those two things. The White House felt totally under siege from, like, that set of questions. I think I have regrets about that in retrospect...

Smith and Klein then go on to claim that they never bought into those theories, and Matt Taibbi has some questions -- and receipts (transcript emphases his):

The Russia stuff, I always thought that was worth investigating. I mean, I guess people can debate whether or not it got too much, and it definitely became a deus ex machina for liberals. I never believed that Russia had won the election for him, and I don’t think I ever said anything that would’ve suggested I did believe that…

Liberals had gotten themselves into a weird place where they wanted some explanation for how it had happened. There was some view that Mueller was going to come out with some report, and that would be the end of this. And I think something very different in the liberal mind is a recognition that there’s something very authentic in Trump’s appeal, and nobody comes in and saves you on a horse. And I think that’s really important. I also think the fact that Trump lost the popular vote, and that the election was so incredibly close in the battleground states in 2016, contributed to this feeling that this guy was a fluke and should be treated as kind of an aberration.

That's certainly what drove the impulse to seize on any fake news in order to reframe what seemed to be an inexplicable loss. However, the fake news that they seized upon was provided to them in spades by Smith's decision to publish the unvetted dossier, without the full context of its origins. The claims in the dossier were absurd, such as weird sexual practices that would off-put most people but especially a known germaphobe like Trump. The timing didn't line up and it offered very little except for rumor-mongering, which is why media outlets had turned it down (CNN, notably, among them) before Buzzfeed launched it and fed a million conspiracy theorists exactly what they wanted. And it produced the effect that Smith laments and Klein disavows ... now.

As Taibbi points out, though, neither man spoke out about it at the time. Quite the opposite, in fact:

I’ve run into Ben maybe once or twice, don’t know Ezra, and have heard both are nice enough guys, but what the fuck? Listen to the reasoning: “Clinton’s close electoral college loss broke the brains of audiences, who to their credit have since come to accept that Donald Trump has enough authentic appeal to win elections. At the time, however, they were so incapable of believing the election had been won organically that they gobbled up any soothing explanation and were reduced to praying for rescue by divine plot intervention. And looking back, that’s what Russiagate was, right? Not that we ever believed Russia won the election for him…”

About that: on July 13, 2017, the Center for International Studies published, “Ezra Klein: Collusion is Likely,” featuring the money quote: “I believe at this point collusion between Russia and the Trump campaign is likely. We know for a fact that Russia hacked into the Democratic National Committee files and released them in ways… designed to hurt Hillary Clinton… Given how razor-thin the final margin was for Trump, we know they might have literally decided the election.” A few days earlier, in Vox: “We know that foreign power conducted a large-scale and successful cyber-espionage effort against the Democratic Party… The election is tainted. The White House is tainted. Our foreign policy is tainted.”

Later that year, on October 24th, 2017, the Washington Post finally broke the story that the Steele dossier was paid Clinton campaign research. That should have been the end of the story, but not to Klein, who wrote in Vox days later: “[Russia] really did conduct social media operations designed help Trump. Both their targets and their timing were extremely sophisticated for a foreign government that has traditionally shown itself to have a poor understanding of American politics… At this point, it would be a truly remarkable coincidence if two entities that had so many ties to each other… and that were working so hard toward the same goal never found a way to coordinate.”

As for Smith:

A last note: in that 2023 Atlantic piece, Smith talked about complaints he received from people like Jake Tapper (who worried the dossier’s publication made the Trump-Russia story “less credible”). “I’d expected that backlash, and at first welcomed it,” he wrote. “I thought we were on the right side of the decade-old conflict between the transparent new internet and a legacy media whose power came in part from the information they withheld. And, of course, I loved the traffic…”

The original title of that April 2023 essay? "After All That, I Would Still Publish the Dossier." At that time, years after the dossier had been discredited, Smith wrote that he would do some things differently, but still defended its publication and its impact:

Don’t you, the reader, think you’re smart enough to see a document like that and understand that it is influential but unverified without losing your mind? Would you rather people like me had protected you from seeing it?

Imagine the alternative, a world in which the American public knows that there is a secret document making murky allegations that the president-elect has been compromised, a document that is being investigated by the FBI, that the president-elect and the outgoing president have been briefed on, and that everyone who is anyone has seen—but that they can’t. This would, if anything, produce darker speculation. It might have made the allegations seem more credible than they were.

I think I'd rather see it with the full context of its creation. I believe we'd have been better off with a media that was interested in that full story, rather than one that went looking for any dirt it could find and either (a) got manipulated by a campaign dirty trick or (b) participated in it. Responsible journalists would have asked those questions and connected those dots, and then reported on the attempted dirty trick as well as the contents of the dossier behind it. 

One finishes this exchange with the feeling that Smith has regrets and Klein is in denial not because they are embarrassed by their actions and decisions, but because those did not finish off Trump politically. They certainly regret the extremism they touched off, and that much seems sincere, but only because it turned out to be so toxic that Democrats turned themselves into nothing more than The Anti-Trump Party for the following eight years. And that has shown no signs of abating. And their very belated entry into the Now It Can Be Told genre, or perhaps more of a Now It Can Be Lamented sub-genre, speaks for itself in terms of credibility.