Gege is smart here he says He is not sure if he could win even if Sukuna wouldn't have Megumis CT so let's analyze this
Okay. Let's go.
First Gojo remarks 10S > Cleave and dismantle it was 10s made him more trouble
That's only his remark if we ignore all context for agenda.
Sukuna relied on 10S to beat him, and Gojo doesn't know about his full moveset, about what other strategies he could have focused on, or even that he could just go Heian Body at any time and become objectively stronger on top of healing.
What Gojo implied is that he lacked the knowledge to make a proper judgment, but wording matters. He didn't say he doesn't know the result, he said "I don't know if I could win even if", wording matters, the implication is that he has no proof, as it didn't happen, but he himself doesn't feel he would do it. Very distinct from his usual arrogance. The wording is clear.
Now, regarding "what else could Sukuna do", of course we don't have a clear answer, as the fight we got didn't go that route, but that's precisely why Gege had Gojo said that, and not "if not for 10S he wouldn't even have touched me".
In fact, other than Open and the times we did see him hurt Gojo early on, there was a moment during the domain battles that Gojo was puzzled Sukuna wasn't using Cursed Techniques when he had the chance to. Gojo already knew Sukuna had other options that Sukuna could be trying to exploit, he just didn't know Mahoraga was actually active and adapting. Gojo wouldn't have been puzzled by Sukuna not using CTs if CTs other than Mahoraga were totally useless to Sukuna in the previous clashes.
That's how soon it was implied Sukuna was simply "picking an option". 10S was just a gamble (that almost failed) to actually grow stronger from the fight and achieving a new technique thanks to Gojo.
Just using his Heian body would have greatly change the Domain Expansion battles from the beginning of the fight.
All if this is even more irrelevant now that said new technique is just a basic part of Sukuna's moveset after learning it.
Second he assumes Sukuna didn't go all out but we don't know what's his allout his and if he has something stronger than cleave and dismantle
So when Gojo "implies" 10S > Cleave it's the word of God, but when he directly states Sukuna didn't go all out it's an assumption? It wasn't backed up by around six other statements from different characters throughout the fight and even currently by Uraume.
His all out is stronger, that's for sure. may even be something he can add to his DE. He won by holding back whatever his original triumph card is.
As you said, Gege is smart: he found a way to give months of fanservice to Gojo, which I loved, yet still made it clear the strongest in history was stronger than the current time's strongest. He even made Gojo go out of his way to cite all of the things he used to the max in the fight just to say "shame I couldn't make Sukuna do the same".
There's zero ambiguity.