co-op, base-building, and mech combat

ah yes, just like Horizon Zero Dawn.

Or is it just the 'humans fighting giant machines' part that they're likening to ~~Shadow of the Colossus~~ ~~Metal Gear Solid~~ Horizon Zero Dawn?

Jokes aside, the standard of "could confuse consumers into mistaking one for another" was meant to prevent things like essentially typo-squatting in product names, e.g. going and making Orao cookies, instead of Oreo (which is why Oreo was able to copy Hydrox).

It wasn't meant to just be about aping a concept or art style. No one would actually mistake "Light of Motiram" for "Horizon: Zero Dawn".

Yes, let them fight.

I say unless they’re using the exact same assets, then who cares. People will play the better game.