...yes... that... is what I wrote...I don't think it's a house rule. That's how layered armour is described as working in the new version: you treat each level sequentially. I was wondering why they called that out, but it's relevant for AP.
I'm going to pretend it was brilliant, rather than just something to deal with various types of armour that are good at certain things (like vs. lasers or heat) and say that it is what I meant for armour piercing ammo as well.
Honestly, I'm a little of the fence with whether that's realistic (I'm thinking of a bullet going through multiple phone books (when phone books were a thing) and trying to imagine if the AP trait follows though fresh to the next layer, or gets attenuated by the first layer.
If this was a Mythbusters episode, there'd need to be an experiment...