You guys are missing something here. How did Leonard manage to avoid seeing the movies and also internet chatter before finally reading the book for that even to still be a spoiler?
Hell, I'm not a Potter fan, but I've at least seen the movies and read online all the major plot points without having opened the books.
I personally found it strange that he didn't know the spoiler already. It's just one of the many details the writers get wrong on the show when dealing with "nerd culture".
They mostly get it right. At least from this nerd's experience. They probably used Potter because it's owned by Warner and would be more recognizable to the audience.