It's a one-page draft of an interpreter for a subset of a language, by one of the main designers involved with it, written to the initial designer. It's written in C, but idiomatically written as closely to the language as possible. Having comments would be pretty strange, honestly.
Agreed. I think it's atrocious C style, it just makes sense for people who write J natively to style the C that way in a context where it will be understood.
There's an unpacked and commented version of it here, FWIW: http://www.kuro5hin.org/comments/2002/8/30/175531/763/1#1