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.

This hits the nail on the head for me.

I mostly enjoyed the books as a kid, but didn’t really maintain being a Harry Potter “fan” into adulthood. Out of the dozen or so “hardcore” millennial nerds that I know really well, there’s exactly one person that I think would attempt to read something like that, and based on the reviews I’ve read, I would bet that she would struggle to finish it, if at all.

There is a somewhat interestingly vocal group of internet adults that insist that the default mode for nerds in their 30s is to be Harry Potter super fans, some of whom lapse. Like we were all raised Catholic and some stray.

I was a big fan of Animorphs at that age too, but you won’t see me going around reading or writing Hork-Bajir/Andalite fanfic. Because I enjoyed it as a kid and then went on to read other books.