Which sort of makes sense, if you buy the idea that somebody who makes a dedicated study to a fighting style can't use that style when they change armor for any reason. Armor-induced amnesia, perhaps?
But regardless, the way that the Secret of Steel works is that armor grants you additional Toughness, which is what you roll when you get hit, but it reduces your Defense score. In other words, armor makes you easier to hit, but harder to hurt, which by all accounts is exactly what it does. Having high armor doesn't mean that arrows and crossbow bolts don't hurt you, it just means that it doesn't hurt as much as it should were you unarmored.
Since I'm currently playing Labyrinth Lord, I've been thinking about ways to put this same sort of system in LL, without totally changing the nature of the game. It's something that bothers me, to be sure, but where to go with it? There's no way that reversing the armor class tables and giving the heavier armor some sort of "damage reduction" would work out all that well- although it's certainly a thought.
Especially if one decided to do the same with monsters, although that can be a more difficult case to decide. In a lot of cases, monsters have "natural armor" in addition to being difficult to hit due to armor plates or incredible monstrous agility or what-have-you. For example, a griffin in this example would probably have the same armor class, since they're not especially tough, but a dragon would certainly have a much lower armor class, since they're not particularly large, but they're very tough.
But would this make tougher monsters even tougher? It works in The Secret of Steel because armor is abstracted to three classes; Light, Medium, and Heavy, with each one decreasing Defense by a little more and increasing Toughness a little more. Would there necessarily be any difference between banded and splint? And would it increase power-gaming and munchkin-ing? I suppose it depends on how it's handled, honestly, but it's something to think about.