Yeah... all the people guessing that the little girl was actually the old woman were on-point, because of course that's how it works. All the other evidence we have from the past 9+ volumes wouldn't support the little girl being Tsukasa at this stage. It'd go against everything we already know about her. There's just too many contradictions for that to work, so it's a surprisingly weak 'twist'.
Having binged up to this point, I can tell for a fact that Tsukasa has to be the older-looking individual in the flashback. There is no other way for this to work, since we already have pretty definitive proof that Tsukasa has been around for over a thousand years, and has, in all likelihood, been '16' years old for pretty much all of it.
It would also line up with Tokiko's age if she was the young girl in the flashback, as others have pointed out.
ToddSama's arguments feel very on-point. He even covered the point about the onagi by saying Tokiko probably has been making Onagi for Tsukasa as thanks for saving her back then for awhile.