I don't think RedHat is trying to stop anyone from supporting RHEL. That is still trivially easy to do as you wouldn't be changing the kernel, just offering support on it.
There is a very small subset under attack by this change and is the subset that is trying to use all of RedHat's work, but up-sell a better kernel that more closely matches mainline but at the same time contains all of RedHat's work on bug fixes that are rolled into dozens of different kernel versions. This small subset is now presumably forced to reverse engineer every single change and security update RedHat does on their kernel. That could possibly be a very costly task depending on how difficult acquiring the reverse engineering is. Time will tell if this change is effective or if the reality of RedHat moving everything upstream makes the reverse engineering easy.