No bruh he directly mentions being the strongest in their conversation. Again, that convo would make no sense if neither character weren't the strongest of their time from a narrative perspective which is about loneliness = strength.
In fact, to support that even, Sukuna calls him and Gojo worthy of Yorozu's speech, something he didn't give Ryu at all (and in fact, bro got one-shotted for it).
Or if Edo Era mfs had both cursed techniques yet Kashimo dominated the scene regardless, then he'd be stronger than people that were on par with... shii I ain't wanna say it. We can't make either assumption though cuz we legit don't know how either is more plausible.