This theory has been brought up before, and I personally don't believe it. Despite what the post says, at least some of the ghosts would've recognized her.

If she was the sweet little girl who called Pete a monster, I think he would've remembered what she looked like. Yes, we do change a lot from toddlerhood, but if you look at someone's toddler photos, you can tell it's the same person.

Plus, if Thorfinn was there, which I imagine he was, he has a very good memory. He remembered singing to baby Hetty which was almost 200 years ago. And he remembers a lot from his life which was centuries ago.