That is the issue with the armor pieces. They don't have numbers that you can verify when people attack you. The weapon ones can actually be seen once equipped because you can view that specific troop with or without museum item and it has the numbers. That being said I assume that it would apply only to the garrisons, tank depots, bunkers, command post, town center troops (But not the donated alliance troops) and forest defenders. When you click on those buildings, it has the word defenders in there.
Because of the nature of them, I'm thinking the generals, machine gunners, ambush traps, donated alliance troops, barracks, factory, and mercenaries camp spawns don't get the bonus because they aren't actually labeled defenders. They are all technically defending but they're not in the defender specific category and the game has always been that way when it comes to labeling. In the library research the factory for example produces a heavy tank when destroyed. It does say it produces a heavy tank to defend. The traps specifically say ambushers not defenders.
Those are just my thoughts and they could be wrong, not sure but the reasoning is sound for this game anyway.
