There still is a bit of a runtime system that you'd write in something like C - you'll see it in the "c" directory of the linked chez repository. Chez is reportedly very fast, it's much easier to use that than write your own from scratch.

I don't know the Racket situation (although I am using their chez fork), but I know that the Idris language chose to move from having their own C based runtime system to chez for the Idris2 rewrite. The reasons that I've seen were that it was much more performant than they could achieve on their own.

I suspect something similar is at play here. That the racket team wanted to concentrate on what makes racket special and leverage Chez for performance.