I'd be surprised if Yudkowsky has read Sartre. But it's a natural thing to do. Harry Potter is (unfortunately) the closest thing we have to a national epic we have these days... well, an Anglosphere epic, but you get the idea.
If this is the sort of thing you're interested in, you might want to read Benedict Anderson's book Imagined Communities.
Am I the only one who sees obvious parallels between Sartre's use of Greek mythology as a shared reference point to describe his philosophy more effectively to a lay audience and Yudkowsky's use of Harry Potter to accomplish the same goal? Or is it so obvious no one bothers to talk about it? Was that conscious on Yudkowsky's part? Unconscious? Or am I just seeing connections that aren't there?