Responses
Yes he didn't and he wouldn't reveal if it's a plot twist kinda thing. I don't even know if he being one of ten is even a thing, (but you know in the writer's world there is this thing like, if there exists a gun in drawer, it should go off by the end of book. A full circle kinda thing I mean.) That's what made me think there could be relationship between that and Yuji's extraordinary strength.
What if that parasite brain thingy is actually Sukuna's brain that kinda survived?
I don't have any theory for it but maybe ehh.
Also what if out of 10 fetus Yuji is one of them, he never had any parents, (that would make grandpa related to Jujutsu world and explain how Yuji was stronger than normal human... He ain't completely human, but took more after his human mother) that's what his grandparent was trying to tell.