They're not changing the license that governs the open source code, they're changing who receives the source code directly from them. The GPL requires that if you distribute binaries based on GPL open source code, you also have to distribute the source code as well. If you modify GPL'd source code and produce and distribute binaries using that modified code then you also have to distribute the modified source code as well. However, the important point is to who the GPL requires them to distribute the source code. The actual requirement in the GPL is that you have to distribute source code to the same people that you distribute binaries. You're not required to distribute source code to anyone and everyone.

For Red Hat's enterprise customers, they'll still have access to the source code that makes up the distro. Source code packages will still be a thing and licensed RHEL customers (including the free-as-in-beer developer license) will still be able to install source packages. Red Hat cannot do otherwise as it would put them in contravention of the GPL license. What is changing is that Red Hat is no longer publishing the same source code publicly. They used to do that on git.centos.org but have now stopped. The general flow of code changes used to work something like this:

Fedora (and now CentOS Stream) -> RHEL -> git.centos.org -> downstream distros (Rocky Linux, AlmaLinux, formerly CentOS before it become CentOS Stream)

By breaking the link at git.centos.org, Red Hat makes it harder for downstream distros to create versions that are one-for-one binary-compatible with corresponding RHEL versions. Doesn't mean it's impossible, and certainly both AlmaLinux and Rocky Linux have put out statements saying that they will work around the problem and continue as per usual.

Hopefully this simply becomes the new status quo. Downstream RHEL-compatible distros have a harder time of it because they have to reverse-engineer each RHEL build to some extent rather than receiving the exact updates directly from Red Hat themselves. However I do wonder whether this is IBM / Red Hat's first step toward an attempt to kill downstream distros, and if there are changes coming to the Red Hat license that make it less free-as-in-freedom. I hope that's not the case because at that point things become very contentious and there will likely be litigation as to whether Red Hat can legally lock down what mostly amounts to a curation of open source software.