The FSF is not relevant here - as the copyright holders of the code they wrote, they could not get in trouble for violating the GPL no matter what they did. I think that if it ever did get to court, showing "the preferred form for making modifications to the program" would be pretty straightforward.
Counsel: And so during your time at Red Hat you normally worked on changes to the software using the git tree?
Witness 1: That's right.
Counsel: But why didn't you just use a tarball instead?
Witness 1: That would make my job more difficult, because...
Counsel: So you would say that you preferred to operate on the git source tree rather than a tarball?