Yeah, its one of the things that's always slightly ticked me off about that guide.
Excavating the history of the internet (mostly ye olde usenet groups) I couldn't find much he had to say about Sourcery, but there was this interview where he said:
By about book four, I discovered the joy of plot. (That was Mort.) I went back a bit with Sourcery [1988], because I knew the fans wanted more of Rincewind. I didn't particularly enjoy writing Sourcery, but it stayed on the best-seller list for three months. And then I said, "Sod the fans, I'll do what I like."
which doesn't make me think he'd particularly recommend it.
When a major sub goes private, that's an awful lot of posts and comments that they have to hide from the rest of the internet. I'm not surprised there's some excess load on their systems with lots of them doing it at once.