In my experience, the situation is invariably noted and rectified when cabin crew complete their checks before the aircraft leaves the gate. In that case, they will ask a few people (usually nearby, or for instance in another exit row if one block has two passengers and another is empty etc) if they want to move. They would never force anyone (indeed, that people must "feel" able to help in case of need should in principle be a requirement to be in the emergency exit in the first place). It is more or less easy depending on whether the seats are in Y or C - so for instance, sometimes, the exit row maybe at the back of a largely empty C cabin, especially on a lightly loaded flight, in which case, C pax may refuse to move and crew may move the curtain and move pax from Y instead who are more likely to appreciate the privilege.

I think that here, the mystery is why the crew who did the check did not see the empty emergency exits as an issue and get it sorted, as noted by CIHY. That's hard to call this anything else than incompetence. Whenever this was noted, the taxiing should have been interrupted really and the situation resolved with some urgency but safely (ie certainly not with the aircraft being on the move but rather with it brought to a place where a full stop would be authorised, which may involve returning to the gate).