After looking at the Racket website, from the POV of someone who wants to use or learn Scheme, that doesn't sound like a useful distinction. They have R5RS and R6RS implementations that look to have reasonable conformance with the corresponding standards. I don't see how the fact that the language called "Racket," which is a lispy language that implements (among other things) R5RS, R6RS, and mzscheme even matters. It sounds an awful lot like like saying "foo-scheme isn't a scheme because it's implemented partly in C."