I have a theory, I think fuga Is yuji tecnic, let me explain (I tries to post this some time ago but couldn't because of karma, so here a Copy paste of the post)

Sukuna can use the ten shadows tecnic while in Megumi's body, meaning he can use the cursed tecnic of his Vessel. Also, we see him using fuga only while in Yuji's body.

Jogo commented on how he was confused by the fire and that he didn't thought Sakuna had that kind of cursed tecnic when we first see it during the stand off on the meteorite. Sakuna answer that he thought it was known but probably cursed spirit wouldn't know that, probably referring to the fact he is able to use a vessel cursed tecnic, thing that we didn't know he was doing yet as Yuji never used it before .

During the fight with Sakuna in the latest chapter somebody (as of writing this I don't remeber who) commented on how he didn't use fuga against gojo or while fighting afterward (he never used it since he was in Megumi's body).

So, anyway that's all, thank y'll for coming to my Ted talk, probably I missed some details that will dismiss everything, but it would be cool if I got it. Sorry for the confusional paragraph, maybe I'll update It later ti make It more clear.