Oh man, the legal battle when Red Hat tries to crack down on "pirated" GPL software is going to be... interesting.
(that's where this is surely going, right? Or else they're just not going to bother actually trying to enforce this policy change, and let all these 3rd parties use alternative methods of obtaining the source)
If I had to guess, simply making these alternate GPL sources unofficial _enough_, will be good enough. If the whole alternate source is stained with the guise of "this is technically allowed but has no warranty and no guarantee it will receive the same patches", many people will jump ship and pay for support. Whereas right now, if it's literally the same thing, there's no reason to.
Think of it this way: The GPL doesn't obligate Red Hat to accept all customers in the first place, right? If you stop paying for your RHEL subscription, the GPL doesn't oblige Red Hat to continue it and continue distributing future releases of RHEL to you.
But even if you stop paying, you still have your GPL rights for the versions you received.
The only difference here is Red Hat saying "yeah, we're not taking your money anymore. Enjoy the source code and binaries you've received so far, but you're not getting new ones from us."