All that the authors write in the New York Times essay about corroborating the story is this: “Mr. Stier, who runs a nonprofit organization in Washington, notified senators and the F.B.I. about this account, but the F.B.I. did not investigate and Mr. Stier has declined to discuss it publicly. (We corroborated the story with two officials who have communicated with Mr. Stier.)”
So they corroborated the fact that Stier made the allegation to the FBI, but the authors give no indication that they have corroborated any details of the alleged incident.
The book isn’t released until Tuesday, but Mollie Hemingway got a copy, and she writes on Twitter: “The book notes, quietly, that the woman Max Stier named as having been supposedly victimized by Kavanaugh and friends denies any memory of the alleged event.” Omitting this fact from the New York Times story is one of the worst cases of journalistic malpractice in recent memory.
If you take this confusing accusation in the essay at face value, it doesn’t even appear to be an allegation of assault against Kavanaugh.