Seriously I got to ask, you honestly think this is anything but a bit of lazy writing?
Like, you really think they went and thought about all you explanations before they wrote that scene?
Again, I don't think it ruined the movie or anything, I mean I do think it's better to establish what your characters can and cannot do, especially when you deal with giant robots and giant monsters, otherwise you get that "oh, I guess they can that now" feeling, which can feel the scene being bit unearned, but that's the extent of it.
I think they wanted to write a scene with a cool super sword that wrecked the shit out of the monster, but they wanted to maintain the tension and keep the robots underdogs, so that's what we got.