This isn't about editing patches directly (which, btw, is perfectly possible and something many maintainers of software projects will have done). This is about the sources which RedHat use for the kernel RPM. The process literally /did/ used to be about patches:
1. the src.rpm was created from an RPM spec file, which specified as input various files, including a base Linus kernel tarball and a bunch of patches.
2. the kernel binary RPM was built from this src.rpm.
RedHat maintainers literally did check in *patches* to CVS, as the source of their kernel src.rpm. Under this process, the patches clearly *are* part of the source for the build. (Note src.rpm is a reversible process, so the src.rpm file and the unpacked files are equivalent, i.e. step 2 could skip creating the actual src.rpm file - the point is the source consisted of patches).
Now, some of this has clearly changed - they're using git. It could they're now using git to apply patches to a kernel tree and building directly from that (as airlied seems to indicate), in which case it seems unlikely the history is covered by the GPL. However, if the start of the build process still works on a patch list, then that would still seem to be source and should be released.