Right, meant to say "the platform that powers this site".

Arc itself is an interpreter written in Racket. The site runs on racket.

The Arc compiler converts Arc expressions into Scheme (in the mzscheme dialect) as implemented by Racket.

LISP people likes to refer a language modification to a "language", partly because its lexical structure is so simple and its metaprogramming makes such modification easy. But yeah, Racket and Arc are only similar in the surface syntax.