That page leaves it clear:
"All fanfiction involves borrowing the original author’s characters, situations, and world. It is ridiculous to turn around and complain if your own ideas get borrowed in turn. Anyone is welcome to steal anything from any fanfiction I write."
He only says you're allowed to steal it. Not to use it with permission. If you take it without permission, that's stealing, so you have permission, which means that you didn't steal it, etc.
You know the drill - If it's worth saying, but not worth its own post (even in Discussion), then it goes here.