I could be wrong but my understanding of the rules are that the ball is placed at the spot it's at when the play ends. That includes crossing the goal line, going out of bounds, when forward progress is stopped, and/or when any body part between and including the forearms and knees touches the ground as a result of or in conjunction with contact by defender.

If they're saying Elliott was down at the point progress was stopped, then he was down before he stretched the ball. If they're saying progress hadn't yet been stopped at that point, then he pulled the ball back before he was down.
Either way I think they got that call wrong.