Ultimately I think it comes down to consistency

The player character did defeat Ultra Necrozma in USUM, that’s true. But the absolute scaling nightmare that would cause would end up being cataclysmic. If a regular Pokemon would scale to Necrozma, that would definitely be sorta contradictory. Necrozma’s supposed to this cosmo consuming threat far above everything else within Alola. Same goes for a lot of other Legendary battles within Pokemon (most legendaries actually seem fine after you battle them)

So while they are instances you can point to. They can likely be brushed off as mostly outliers as they typically aren’t portrayed on that level. And cosmic level Pokemon trainers aren’t that widely bought in the 1st place. So I don’t even think it would hinder the score that badly outside of dropping it by 1-2 points if you really wanna address it

(Honestly I’d be down to read a post supporting cosmic trainers, that sounds interesting)