They didn't charge him with failing to report the transaction.
Is three a magic number? Why three? Why not five? Or two?
I can easily imagine building up a few 80% guns, realizing I didn't care for them (tuning, I did a crap job, not into ARs or "Glocks" anymore, whatever) and selling them, even all to the same person who thought they could make them work right or have fun with them.
Easily asserted, sure. You're assuming (probably correctly) that this guy was making them for resale. But how do you prove that? Do you have to keep them for a magical period of time? Do you have to shoot them a certain number of rounds? It'll probably come out in court that the cop said, "can you make me three?" and the guy said, "sure!" and he'll get convicted. But his dumb-ass speaking isn't the same thing as it obviously being a felony just because he made a/some 80% guns and sold them.
It's bad optics.