You can see in multiple of the replays throughout this thread that it lifts the entire groin guard with the force of the shot. A groin guard in Boxing isn't like a metal egg shaped cup you slip into your shorts, it the protects the whole genital area as per
@randomg1t point about avoiding the explosion of your nuts.
If that piece of equipement gets uppercutted at the top then its gonna force the whole thing to ride up, giving your nuts a big ol' whack. The idea that Uysk was intentionally faking or lying about this shot having caused pain in his nuts doesen't hold any water to me.
The ref should have been clear about the belt line in the pre-fight instructions in the ring but Barry Jones is completely correct to say that, in the absence of any special instructions by the ref, if the shot lands on the groin guard then it illegal. On my first couple viewings I though it was more borderline as to how bad it was but from different angles I think it obviously lands square on the groin guard and lifts it up. I've been hit with shots like that and believe me you're going to feel as though someone has just dinged you straight in the testicles immediately afterwards.