--I barely got through the animated forest, found a room full of statues that started shooting fire at me and hoped that if I went back into the animated forest I could be able to lose them in the trees, but I didn't think about the Fire vs. Tree thing. When they started shooting flames around and killing the trees I then assumed 'since I used burning hands on them last time and got swarmed, these statues should get swarmed', but to no avail.

Shouldn't mobs fight back against other mobs if they are getting attacked, especially if they are animated trees getting lashed with fire?

Since the trees don't attack the statues, this could be exploited as a farming strategy (in the rare event such a flame-throwing mob is found and dragged there, similar to the banshee method that was fixed).