In fact, the FSF says it is GPL compatible, which means they are not only free to fork LLVM if they think Apple's stewardship of that project is risky, they can make their fork GPL.
Another thing to consider: Apple employs the majority of LLVM developers, including the lead developer. If Moglen is correct, why is the new code these people are contributing, as Apple employees, still under the old license? Wouldn't Apple be putting their new code under a less open license? And wouldn't they be getting the authors to relicensing the existing code under a less open license, or rewriting it?
The fact that Apple continues to put a lot of work into LLVM under a license that is very open (essentially a BSD license) is inconsistent with the motives Moglen ascribes to them.