If she went in to the water that side of the weir, then yes, very possible.
PF is saying that in his professional opinion, she wouldn't have been carried over it. And I assume (or at least hope) that it's underwater structure has been thoroughly checked, so she is not caught up in debris next to/under it.
We are tending to assume that she was gone by 9.33. But if there was foul play, she could still have been nearby but overpowered. And after that passerby had gone (and the dog no longer loose) there was quite a stretch of time before anyone else seems to have gone along that way.