I think that Eliezer is very well-situated by both age and social niche to find HP fanfiction vastly more appealing than most human beings do. I think that he genuinely finds the idea of "Harry Potter but more as a hard fantasy with a hero bent on breaking the setting by using everything without regard for genre convention" interesting as a story concept. And when he set out to write up his ideas about rationality in the form of something other than dry nonfiction essays, I think that HP fanfiction sounded good to him.
I don't think that he correctly found the medium that is maximally appealing to the potential target audience for his ideas (like, I definitely do not want to read a super long Harry Potter fanfic, and I think I'm joined in that by essentially everyone who's not a very geeky millennial), and I don't think that he gritted his teeth and wrote a HP fanfic in order to increase the audience for his ideas despite not liking HP fanfics.