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.
(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)