i think there’s an underlying meaning to what eren’s mom said here. she might have known that grisha was a titan. i mean what exactly was eren before he was injected with the serum? 100% human? maybe what she meant is that the fact that eren is grisha’s son makes him great because no one knows what can become of a child who is born from a human and titan. if i’m right, uri (historia’s uncle) did not have any children and the next one in line to take the “power” was frieda. assuming that uri was born before his father inherited the king’s first will, and that was the same case for the predecessors of the reiss family, then eren could be the first and only person who was born to a human and titan. it is also possible that only people from certain bloodlines can transform into titans through the serum (i’m taking into account that the memories of the “minority” cannot be altered) but connie’s mother also seems to be a titan so this idea might be a little far-fetched. (posted this comment on the discussion board for chapter 71 on myanimelist too lol just so you guys know that’s me and i didn’t plagiarize)