I suspect they are exaggerating, or that the claims were misunderstood. Yes, the FBI can find criminals who use Tor, but with extremely few notable exceptions from many years ago now, that isn't done by breaking any of Tor's protections, but instead using more traditional police investigation methods (e.g., "we got a tip this person is up to something, and this account only ever posts when that person is at home..."). We know this because details of investigations are public once they go to court in the US. Yes, parallel construction is a thing, but the idea that they regularly make use of exploits and this information never gets leaked in any sort of verifiable way (even with FBI agents who are apparently willing to speak of such secret programs with mere acquaintances ;)) means we can be relatively confident that, if such exploits do exist, they are rare enough to be too expensive to waste on anything but the most extreme circumstances. In other words, no, the FBI does not know what you're doing on Tor (assuming you're not at the far end of a bell curve, and ignoring everything other than what Tor protects).
