I agree that Flynn was railroaded, but McCarthy completely overlooks an obvious point here (which is surprising because he himself is a former prosecutor).
The FBI had the recordings he describes, but since they were obtained through a FISA counterintelligence warrant they would not be admissible in court for a criminal prosecution. This is why the FBI had to obtain independent evidence of what they already knew from the recording.
I suspect this is why Flynn's sentencing has been delayed at least three times. If the only evidence of his "lying to the FBI" is a recording obtained through a FISA warrant, then that's one thing. But since we're now learning that the FISA warrants to conduct surveillance against people connected to the Trump campaign were obtained illicitly (and maybe fraudulently), then the underlying evidence against Flynn is tainted and he has all the legal standing he needs to vacate his plea deal. I'm sure Mueller knows this, too -- which is why Flynn hasn't been sentenced.
Why can’t Flynn see the evidence that implicates him? That includes all unredacted FISA documents and communications.