The author never said that Yuuji was one of the ten
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.
The author said that the others weren't high level curses so he wouldn't show them so, although I understand your POV, I don't think that's the case because Todou would be somehow related to them and that wasn't show either and it wasn't Yuji's ability or a technique -according to the author-
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.