The guy who asked to change seats is why she insisted he move back to his seat.
Which guy who asked to change seats? The other guy said he had no problems with the change. I don't think he was asking to be able to return to the other seat. If it was mutual, why didn't she let it go ahead and just get on with the boarding?
If he refuses to follow her instructions in a calm situation, I imaging he will be even less inclined, in a smoke filled cabin during an emergency.
That's a long bow. No one knows how anyone will behave in that situation. The guy could just as well be the hero who saves some people. I imagine all those pax who retrieve and carry their cabin baggage down the slides in the many videos of real life evacuations, despite being told during the brief and more than likely during the evacuation explicitly not to were all rule abiding pax at the time of boarding too.