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 the Rocky people 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.