I am not sure what you mean by "standard practice of using C as an IR". I was a compiler engineer for a decade or so and I have only seen C used as an IR for quick and dirty hack projects which don't care about things like debugging or compile time. The commercial compilers I have worked on each used their own intermediate representation; none used C.
Apparently they include an interpreter as well which probably could be used for debugging. And hopefully interpreter behaviour matches compiled behaviour, then ;-)
