From the Author's Note:

Now this story has a plot, an arc, and a direction, but it does not have a set pace. What it has are chapters that are fun to write. I started writing this story in part because I'd bogged down on a book I was working on (now debogged), and that means my top priority was to have fun writing again.

From Kaj Sotala:

The other reason is that Eliezer Yudkowsky showed up here on Monday, seeking people's help with the rationality book he's writing. Previously, he wrote a number of immensly high-quality posts in blog format, with the express purpose of turning them into a book later on. But now that he's been trying to work on the book, he has noticed that without the constant feedback he got from writing blog posts, getting anything written has been very slow. So he came here to see if having people watching him write and providing feedback at the same time would help. He did get some stuff written, and at the end, asked me if I could come over his place on Wednesday. (I'm not entirely sure of why I in particular was picked, but hey.) On Wednesday, me being there helped him break his previous daily record on amount of words written for his book, so I visited again on Friday and agreed to also come back on Monday and Tuesday.

Eliezer is not "busy writing his Harry Potter fanfic." He is working on his book on rationality.