Hey, disagreeing with myself is a favorite pastime of mine.
Let's see.. Ablative armor is news to Captain Benteen in "Paradise Lost". Doesn't mean it would be new, though. She (and Starfleet Operations) is just ill informed, is all.
Benteen also believes somebody "equipped" the hero ship with the armor. But since she and SF Ops are out of the loop, there's no way of telling when the equipping took place. And Benteen is being sarcastic anyway.
Ablative armor first explicitly saves the day in "Way of the Warrior". Doesn't mean it wasn't in place previously, though.
Naah, can't find a way to disagree. Sorry! In any case, I never disagree with me. See above.