[polishprince]

But if the seats were empty and they weren’t sold, assigned or “given” to someone else there should not have been an issue. Also if another gentlemen offered to exchange seats there shouldn’t have been an issue either UNLESS the doors were about to close and the shuffling would have delayed the flight. I have seen that happen numerous times where the FA was asked if a person could move to an empty row if j one was seated there.
As someone else said the article left a lot of info out. In my opinion just from what was written the FA was wrong.

[CaptainK]

But if the seats were empty and they weren’t sold, assigned or “given” to someone else there should not have been an issue.

If there is an empty seat in first or business class and someone purchased an economy ticket, I don't know of any airline that would allow that customer to upgrade-move their seat for free.