GoldenOsiris
The reason as to why he's actually holding back is because his other abilities like box and what not, are useless against Gojo with his infinity. He literally has no way winning when we know that he uses Maho to bail him out from UV or the BF stun, or him being nervous. He also admits in chapter 236 that he needed a model in order to even bypass infinity, as in that was his only way of winning. Without the 10S he is bound to be hit by a HP more earlier, as nothing can bail Sukuna out by then. The things that we have seen him currently use right now, is his original body (there are no statements about it being physically faster and stronger than the previous), that special grade tool with the ability to use electricity and such. Though said tool had to be delivered to him via Uraume lol. His box ability with the fire thing, is also useless against Gojo when it cannot bypass infinity. It is also stated that few can survive the fire arrows, and much likely gojo can survive it too, so I doubt that could do anything either EVEN if it bypasses infinity supposedly. All in all, Gojo would definitely win. The only reason as to why he is unsure, is because he is ignorant to Sukuna's abilities, and bases off his unsure guess from Sukuna using that space cutting cleave too, unaware that he needed Mahoraga for it. He's also saying this because he doesn't understand Sukuna's abilities. Otherwise if he knew about the box, true transformation (which only helps you back up lol) and that tool... He would be more confident. (We don't know if Sukuna has more stuff in his arsenal though, but he again says that he needs mahoraga to win in the first place lol)