I assume they know but don't care. Either way, that is a bad choice. I think "Rad" would be a good name, but maybe they already are using that for something else.

Edit: where did you see it's called "R"? It looks like they call the system language "Radiance" : https://radiant.computer/system/radiance/

I assumed R and R' are prototypical bootstrapping variants of what will be the full-fledged Radiant language, but that wasn't explicitly written anywhere.

well-known "language" (air quotes)