Oh, there's worse things Red Hat can do to block access to GPL source. For example, technically they only have to provide source to their customers, and there's nothing in the GPL that stops them terminating the contracts of any customers who distribute that source code and closing off their access to updates.

In a way they're already doing that with kernel source; the actual broken-out patches applied to the kernel are only available to customers and they're contractually obliged not to distribute them otherwise their contract will be terminated and they'll lose access to support, software and security updates and the right to run RHEL at all.