It's written in mzscheme. I don't know exactly what self-hosting means, but PG did write his own web server, and he said he very much wouldn't want to deal with Reddit-sized traffic, so I expect he runs everything on his own computers.
Self hosting = interpreter/compiler for the language written in the language itself.
It "compiles" into mzscheme. But the compilation is more like macroexpansion.
Ok, this truly piques my interest in Scheme. (If only we could get our hands on ARC!.)