EDIT: they might actually be the original author of that? I can't find this indexed anywhere else online (google, google scholar, and google books all turn up nothing or just that mastodon post)
Now that I'm looking for it, I can't find it anywhere, I think it might just be something unpublished from the person on mastodon. Would make sense with them saying they love footnotes
Not necessarily. Self citation is different than building on your previous work. You might just seek to use other citations for the relevent concepts
Edit: the 2015 paper this is referencing lists many differing potential reasons for it. Ranging from worrying more about negative feedback for self citation to being more likely to being more critical of their own work