1) Armor is not "damaged clothing" and should have no means by which to ever BECOME damaged, thus Toady's quote above should never apply to this situation.
Armour is handled as clothing with an extra flag ("armour level"). I'm pretty certain that this level both restricts equipping to the military menu only and at the same time removes wear while equipped. That armour in refuse piles is treated like _any other wearable item_ is not very surprising, although it feels like a bug. (By the way, leather and wooden armour can get damaged by excessive heat or cold; metal armour would not take damage but rather melt).
Just being equipped still doesn't seem to make armour wear out - i have a bunch of spare time militia who've been wearing their iron mail shirts for years, and they're still in pristine condition. I could imagine that use (i.e. combat, including simulated in sparring) could cause armour damage, but haven't heard of anything like that, so accidentally throwing freshly-forged armour onto an "everything including refuse" pile still feels like the most likely suspect.
2) I'm pretty sure furniture and other stuff rots in a stockpile with refuse included,
Undamaged furniture certainly doesn't. I haven't seen anything decay through the x/X/XX symbols that wasn't clothing/armour (rotten food seems to rot away faster than otherwise when in a refuse stockpile, but it doesn't go through the visible wear grades; last time i checked, unrotten food didn't rot in refuse stockpiles).