I'm not sure it is necessary for the author also post the link to the tweet that referred to the tumblr post, since the tumblr post is where the actual proof is.
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").
> For example, suppose you are reading an article by Brown (2014) that cites information from an article by Snow (1982) that you would like to include in your essay. For the reference list, you will only make a citation for the secondary source (Brown). You do not put in a citation for the primary source (Snow) in the reference list. For the in-text citation, you identify the primary source (Snow) and then write "as cited in" the secondary source (Brown).
Regardless of one's preferred style guide, it is odd not to credit the author whose work that led to one's discovery of the inspiration for one's own work.
[1]: https://guides.library.uwm.edu/c.php?g=1007936&p=7303662
Besides, the Tumblr post is already a secondary source.
That's not so you can give credit to Brown for helpfully pointing you toward Snow. It's a requirement that you admit, when you cite Snow, that you never actually read Snow. If you read Brown, find a pointer to Snow, and then read Snow, you don't cite Brown at all.
"A favourite proof of mine: first demonstrated to me by my professor in number theory. I think its beauty stems from the fact that it requires no knowledge of mathematics above the definition of what it means for a number to be rational, and can be written almost in one line."