Well, that'd be true for 9mm vs. Kevlar if all your shooting is fmj, but any expanding round is gonna fail to penetrate unless it has a hardened core as well.

That said, I don't know too many tough guys that would stand up to being hit in the torso with 10mm or 45, whether they're wearing body armor or not. Likely they are gonna get knocked down regardless, even if it doesn't penetrate, but 9mm ammo doesn't have the thump factor unless you get the velocity up around 357 mag full house ammo.

Let me put it another way. Let's say the 200 lb maniac wearing level 3 is running toward you from 25 yards out full tilt bozo. You can empty all but one round from the mag on a high cap 9 into his torso, putting all hits in the armor, and he'll definitely be feeling the hits that will sting like a sonofagun, and you might slow him down enough to squeeze off that head shot at 10 feet. But you are not going to stop his forward progress up to that point unless one of the rounds penetrates the armor and hits something imminently vital, and that's not going to occur unless you're shooting fmj ammo.

That same guy is either gonna get knocked over backwards or dropped to his knees if you do the same with 200 grain JHP 10s or 45s. It is irresistable force being applied, and he just doesn't have the mass to overcome the inertia in the rounds hitting him.

Here's another perspective. Throw a baseball hard as you can at a guy wearing body armor, hitting the armor squarely. There's a lot of energy in that baseball, but it doesn't push him around. Now take a bowling ball and do the same thing. It is going a lot slower, but he can't help but be displaced.

Hopefully that helps.