My only guess is that the rebels worked in cells, Leia's cell heard about walkers but never faced them, whereas Mon Mothma's might.
Pretty sure Leia saw AT-ATs before (see: Star Wars Rebels episode: A Princess on Lothal).
Or the fact that an AT-AT isn't fundamentally different from an AT-TE from the Clone Wars.
I'm guessing we're not supposed to overthink any of this as X-Wings work in the absolute zero of space, and they work in normal atmospheres, so there shouldn't have been anything different about Hoth that would have prevented them from attacking an AT-AT. And if a squad of X-Wings can take out a Star Destroyer than I would think they could take out an AT-AT.