So basically, the theory comes from Yuji telling Sukuna to try one more time but without cursing people, saying that even if nobody accepts Sukuna, that he will live with him. On top of the fact that Sukuna said that if there is another time, that he'll walk down another path. Plus the themes of coexistence that are pretty prevalent in Modulo.
The theory basically says that Yuji is hiding because he's raising the reincarnation of Sukuna, and if everyone knew about the reincarnation thing, they would probably hunt him down.
I don't know how true or how realistic this theory is but I would personally not mind it at all. But I believe that it would be pretty sick to have JJK 1 as the "background" of Sukuna and JJK 2 as the story where he redeems himself.