Exactly, the demoncrats are intentionally confusing the issue. "Armor piercing ammunition" used to mean steel or tungsten cored bullets intended to penetrate steel armor. The demoncrats are attempting to expand that definition to include any ammunition that can penetrate soft body armor. The highest level of DOJ soft armor protection, level IIIa, is only intended to stop handgun and some shotgun ammunition. This amendment could effectively ban all center-fire rifle ammunition, since any bullet (including jacketed soft point or even cast lead) with a velocity over about 1,500 feet per second can penetrate soft body armor.
That tactic work well with the term "assault rifle".