Board Thread:Manga/@comment-34196931-20190808205129/@comment-1327106-20190809211642

Cheng The Nerd wrote:

second, girl in the titan realm is Ymir Fritz herself confirmed.

I wouldn't say it's confirmed. It's simply Zeke's conclusion. Eren asks "How do you know" and Zeke's reply is "Who else would be wandering this place but her?"

Basically, Zeke doesn't have a better answer, but believing she's the person all the Titans came from is the best idea he has.