Well, according to a mathematician friend of mine, he definitely did not. He went to a lot of trouble later in his life to prove things that would have followed

Well, according to a mathematician friend of mine, he definitely did not. He went to a lot of trouble later in his life to prove things that would have followed trivially if he had had such a proof. Besides, the result is interesting enough that he obviously would have published the proof if it existed. Most likely he thought he had a proof when he made the margin note, but later found a flaw. He would have felt no need to issue a retraction, since he had never formally claimed to have a proof in the first place.