I don't think there's anything in that comment to indicate that they didn't do it that way. There's a robust discussion about this elsewhere, but people making embedded DSLs is basically routine. They just don't always call them that or have awareness that what they're doing fits that description. But almost all of those will be written in the "host language" of just whatever the containing system happens to be written in.

Some languages are particularly suited for this but other than racket and ruby it seems mostly accidental/incidental to their design.