We can debate the legitimacy all day long but the fact of the matter is that if someone pulls the chord when they have simply not paid attention then they could expect a fine, and I don't think anyone would argue against a fine for such a misuse and I don't think anyone would condone such a misuse.
But I would be extremely shocked if anyone would be prosecuted in the event described by the OP. I personally don't think I would do it, unless there was a really urgent need e.g. a passenger was unwell, or it was the last train back, or it was a very long distance to the next stop, or some other urgent reason.
I do find it difficult to believe that the event happened, but we have to assume it did. I would urge the OP to ensure it's properly reported as an OMTR download could conclude if the stop was made and if the doors were released and if not the guard would have to explain to his superiors.
Such an event must be incredibly rare, almost unheard of.
I don't want to suggest it's the case here, but passengers do sometimes tell 'porkies' to people meeting them. The other day I was on a train to Nottingham, someone asked if the train was on time, he was told we were 3 late. He then said "the train is late" on the phone, yet we were early into Nottingham. OK, so that's a slight exaggeration but I've heard much worse than that! It is feasible at a stop like Glenrothes for someone to expect the doors to open on the opposite side when heading East. Someone making such a mistake may be tempted not to admit this blunder.... If the OP writes to FSR detailing the allegation, if they get a letter back insisting the doors were released, then that's possibly your answer.