When writing harry potter, Rowling had to craft a world with characters and rules, make it coherent enough and tread a compelling story through it. She made the legos and then crafted something with them.
This guy came to her crafted world, and moved around a few bits, changed a character here, change a few rules there - but the backbone of the work was made before.
It's just not the same ballpark
Lest you think I have an ax to grind with Yudkowsky, I did enjoy Three Worlds Collide. It shares many problems with HPMOR (the writing is inconsistent and the world-building is incoherent) but it at least tells a tight story with a beginning, middle and end. I’d say it’s about as effective as the median submission to Amazing Stories back in the 60s.