I think that "one shot" was a fluke and we did
slightly overestimate the gap though. Although I don't think Marco would push Big Mom to mid or high diff per se. The implication was more like Marco being a terrible matchup due to his flames directly affecting her homies, kinda like Brook's powers tbh. We still see her casually grab Marco like it's nothing.
In WCI, we were also told that an enraged Big Mom would easily kill off her entire crew. In Marineford, we see Akainu fight all of Whitebeard Pirates' Commanders and Crocodile solo. And we see Kaido take on a guantlet of multiple YC level and above opponents and is only just starting to show some weakness.
I guess the implication behind that is that a top tier, Yonko/Admiral can solo an entire Yonko crew by themselves in a high diff fight.
I'd say that every Yonko would be able to low diff their YC1s.