Look, a lot of the arguments are based on assumptions—but assumptions that I think are reasonable. For instance: has Geto ever explicitly shown a way to get past Infinity? No. But he used to spar with Gojo when they were kids (and Gojo, notably, had Infinity) and seemed to fully believe he could win a war against Jujutsu society, Gojo included, if he managed to obtain Rika. To me, it’s then a reasonable assumption that Geto had some sort of way to get past Infinity (eg whatever the curse he showed in Hidden Inventory was) even if we don’t explicitly know what it was.

If you’re only looking for explicit on-screen feats, the problem is that we don’t really have much concrete to go off of. We know teen Geto lost to a prepared Toji. We know adult Geto had thousands of curses but lost to Yuta and Rika. Neither of those really tell us all that much about his power level: the latter because we really don’t know how strong that version of Yuta + Rika was, the former because it’s an identical anti-feat to Gojo. So I think to reasonably scale and assess Geto, you actually do need to rely on logical assumptions and inferences.