I generally like it, but as it's been pointed out, to make Naga look smarter they make everyone else a moron.
I mean, IF this was magic on the level of "a single fire spell kills 100 humans", then it'd be logical for the witches to be ignorant about tactics. What tactics? A single spell decimates an army. But that's not the case. The golem is their main defense, and they NEVER tried to find other ways to use it?
Also, I understand there's always a before and an after. The first humans irl to see a siege weapon must have been surprised too. But I doubt THAT was the first battering ram to ever be built in that world...