That would be perfectly legal under the GPL, right?
It doesn't say you're not allowed to modify the code, just that you have to distribute the modified code in its entirety. If what Apple is distributing is sufficient to build (their version of) bash, it's perfectly fine.
If the code is what Apple used to build what they're distributing, then yes this is valid. (For this case at least, they have a tendency of doing this a lot and I suspect the change to bash was made just to address this specific complaint while missing every other place they strip out code.)
I tried to rebuild bash a while back on macos and couldn't do it because the source code was not available for rootless.h. I guess someones complaint finally got some action from apple.
I remember doing macos kernel debugging (in the non-gpl code apple distributes) and it was pretty evident that the code they distribute like xnu is NOT the code that is in the OS. Stack traces had lots of functions apple keeps to itself. Looking in the distributed code shows suspicious blank lines where automated tools must remove apple-only source.
It doesn't say you're not allowed to modify the code, just that you have to distribute the modified code in its entirety. If what Apple is distributing is sufficient to build (their version of) bash, it's perfectly fine.