I think they are talking about not the ability to punch through armor, but rounds made specifically to go through armor. A rifle round will definitely punch through a soft vest, legal. Armor piercing FN 5.7 rounds made for punching through armor, illegal.
Interesting. After a quick search, it looks like armor piercing ammo for handguns is (somewhat) illegal. Armor piercing ammo for long guns is a-ok. Guess that’s why I’ve never had a problem finding it.