I don’t think that is the real Arya Stark. I don’t think she is who she seems to be. The Waif? Recall the scene just after the fight in the Dark Room. Time will tell but to me there is something fishy going on with that character.
If she wasn’t Arya how else would the hound, her and Beric Dondarrion would have know that Dondarrion was on her list?