There is no way Fermat could have fit that in the margin. Definitely vindicated.
My strong hunch is that it was a joke - he knew how difficult the problem was and claiming he had a solution was I think a huge motivating factor for many mathematicians trying to prove it. The greatest nerd snipe troll in history.
In a sense, the proof is a demonstrator not an end in itself. To mathematics enthusiasts it is significant. To the AI it is Tuesday.
Enjoyed that idea. Not sure how true but it was enjoyable.