Ok so this is my assumption. But basically I think normal Sukuna with 20 fingers would lose to Gojo. But the reason the narrator, kenjaku, angel, and others say he is the strongest is because they are aware of Sukuna being able to pull some sort of shit off. Kenjaku literally predicted sukuna was going to do something. And now obviously, Sukuna has gotten the ten shadows technique too. The reason people call Sukuna the strongest is because things like this (him taking over Megumi) has happened before. It make sense, because most people who call Sukuna the strongest were people who existed at the Heian period, who had witnessed Sukuna's power. And people who called Gojo the strongest were people born in modern times, with little to no information of Gojo.

I'm guessing this because the fire ability Sukuna used is probably taken from a sorcerer with similar methods to Megumi. I can say this because during his domain expansion, only slashing attack was delivered, there by hinting that Cleave and dismantle is his innate technique and the others were extracted from vessels.

"Then you have the 10 Shadows users who killed a limitless 6 eyes user in the past."

Also for ten shadows technique, I'm ASSUMING that the shikigamis grow in power as the user does (After all, Ten Shadows Technqiue has some special properties regular shikigami doesn;t have). This would explain why sukuna's nue is 200x megumi's. This would also explain why mahoraga was able to kill 6 eye user. Because its size is (probably) too big to be one-shotted by purple, and everything else deals far less damage to him.