I think the grandparent is under the impression that the alleged Twitter version of the post had a non-anonymous author and that the Github blog instead chose to link to a Tumblr blog, where the blog author claims they learned the proof from an anonymous number theory professor, thus laundering out the authorship.

But I suspect the alleged Twitter version was just a link to the Tumblr, for which it's clearly not necessary to attribute the retweeter (especially in the context of the post being "the proof is actually incomplete, not very good, and less beautiful/more complex than the canonical proof").