Scaling off Saitama has always been weird. It's a weird mix of common sense and headcanon. It's like sometimes the general premise starts off making sense, but they try to make it apply to more and more stuff.
"Saitama was bored with this guy, so we don't need to upgrade him." Eh, valid. Kinda. He was bored with Orochi, but that doesn't mean Foddermon is equal to Orochi or that they're in a similar tier, for that matter.
"Saitama said Boros almost seemed like a real fight and didn't say anything like that to other monsters!" Yeah, kind of valid. But are we gonna then say that every monster, regardless of feats, is weaker than Boros because of one statement? It's always been my personal belief that Saitama said that to either hype Boros up, and/or he began feeling pessimistic about opponents after the person that was "almost a real fight" died after a serious punch. I wouldn't even call Saitama not getting excited fighting Garou in the manga a counter argument to my statement since Saitama wasn't trying to kill Garou at all per Tareo's request rather than the context of the fight between Saitama and Boros, but I digress. The point is that one statement from a guy who's stronger than everyone doesn't automatically mean that Boros is stronger than any other opponent Saitama goes up against if he doesn't praise them. He doesn't have to do that, you know.
Those are just a few examples. Like anything, it's case by case. Mini-rant over.