RowBoat6123

Amusingly enough, it's personal communication with one of his students.

I believe it was his thesis and I assume it was tongue in cheek.

Realistically, the proof would probably just be rewritten in a paper in standard notation, with an indication that the original proof (or the idea) was due to an anonymous author.

You see very similar things happen from time to time where there are "folklore" theorems in mathematics that someone eventually writes down 20 years later. Sometimes they're able to give an attribution, and sometimes not. But always they give a formal proof using modern notation and credit the idea with whatever fidelity they can.

Bill Thurston cited personal communication with himself, which is a pretty unusual source.