Flynn was pressured into pleading guilty to perjury (actually, a near equivalent) in order to spare his son a prison sentence. Yet the supposed perjury itself, as we now know, was not considered a lie by the FBI agents whose queries precipitated the allegation. Moreover, the subject about which he supposedly perjured himself, the conversation with the Russian diplomat, already had been deemed by investigators as containing an “absence of any derogatory information or lead information.”
In other words, they weren’t sure that he lied, and they didn’t think he had done anything wrong in the first place that he would want to lie about.
Even worse (yes, worse), the “lying to the FBI” was a setup from the start. In common parlance, it was a classic perjury trap. Indeed, FBI Deputy Director Andrew McCabe deliberately dissuaded Flynn from having attorneys present for the conversation in question, thus making it sound like an ordinary national security briefing rather than an interview with Flynn as the potential target of an investigation.