Obligatory IANAL, but I think they can't really make it closed source, because it's a whole bunch of code they don't own the copyright to that's under the GPL.
For Red Hat customers and partners, source code will remain available via the Red Hat Customer Portal.
Which I think means they would just need someone who is a RH customer to share the source with them or pay for a license themselves. The GPL really only requires you to make the source available to your customers, not necessarily publicly available to anyone, but it still explicitly allows any of their customers to redistribute it freely.
Though, maybe I'm not fully correct on that, because that would mean that this basically accomplishes nothing besides making RH/IBM look bad.
I'm curious if that's actually true. Refusing to let them specifically buy it, because RH knows they want to re-distribute it, is almost like an implicit extra condition that customers aren't allowed to re-distribute, which is explicitly a violation of the terms of the GPL. Though, of course leaving that term unstated would make it hard to prove.