This is where things get a bit complicated. Their development environment (Compilers, linkers, code/asset library software, the Creation Kit etc.) will all run on PCs. However, the primary target platform is almost certainly Xbox, because that provides a comparatively uniform platform to base code optimisation on and in some respects has more rigid technical limitations to be worked within. I suspect they will, in parallel, have a process for testing and optimising the base code on a variety of PCs, and making what enhancements make sense (i.e. not more work than they're worth).
The quote about the game being authored on PCs might well be true, but might not actually mean what it seems to say.