Somehow I doubt that an at-at can be damaged by a blaster, whats gonna happen in regards to this?
Hopefully nothing. AT-STs, tanks, and deployed turrets wouldn't really be damaged by blaster pistols either, yet all are vulnerable.
Withering Fire means everything can take on everything, including AT-ATs. I don't see how they should be different from a tank or any other vehicle in that respect.
"One of the bitches actually gave birth while she was attacking, and her puppies joined in on the carnage."
--the awesomeness that is Boatmurdered.