It's not really an example of Stigler's law because it was Fermat who came up with the theorem while reading the sum of two squares problem in Arithmetica by Diophantus.

Diophantus provided a solution for finding the sum of two squares but Fermat wondered if you could do the same thing with a cube. He wrote that it wasn't possible for a cube or any greater exponent, and that he had a proof. Unfortunately he was making these notes in the margin of his copy of Arithmetica and his proof was too large to fit.

Diophantine equations is a broader category which Fermat's last theorem falls into.