The same copyright law can be used to enforce the terms of the GPL.
And if you don't like a code of conduct, you can equally just stop participating in a community. Unlike rejecting a license, you can even reject a code of conduct and continue to use the software produced by that community.
This describes, as far as I can tell, the de facto enforcement practices of ONE GPL copyright holder. I don't see where it places any binding limits on even the enforcement the SFLC would take, and of course it has no power to enforce any limits whatsoever on other GPL copyright holders.
In the particular case of Kernel development, contributors are NOT required to assign their copyrights to any shared institution, are perfectly within their rights to take any enforcement action against violators that they wish, and sometimes do exercise that right: https://www.zdnet.com/article/linux-beats-internal-legal-thr...
Stallman is his own man and can boycott what he likes and explain why to others.
"Punitive" "teeth" as found in proprietary EULAs and various unreasonable copyright laws looks like jailtime and financial ruin.