Its not a question of "hey, this will kill the raging maniac" but more along the lines of "this freak has a tough body, so I should use some other tactic than just stabbing or fireballs." And its not like its a huge secret that certain tactics tend to work well on the more physical beings than magic-oriented ones. It just so happens that the Zealot happens to fit the generalization. Really, it should be common sense - there's a reason why the enemy has these abilities in the first place, and they're not too dumb to realize how to best use them.This is a grey area with me. Yes they have the ability to do these things but do they know that doing so will kill them?
So, when the enemy in question uses these tactics, it just so happens to be the answer to the problem without realizing exactly how everything interacts together