I believe the explanation here is that the FSF/GNU is willing to support their tools on non-free OSes as long as those tools work the same as the do on free OSes, and in particular, don't work "better" as compared to the free version because the non-free OS provides additional services somehow.
They don't want their tools to be superior or have better functionality on a non-free OS.
For example, emacs wouldn't take a patch that hooked into speech recognition provided by the OS, unless that was also available on a free OS. And say Microsoft or Apple provided powerful functionality for debuggers via a system api (and this same functionality isn't on a free OS) - gdb would not take a patch that used it.
And while I have as of yet no opinion on the concept, a lot of people believe he's hypocritical for supporting non-free OSes in GCC and Gnu Emacs.