Describing people using Git without GitHub as an "edge case" is arrant nonsense. Git was developed for the Linux kernel, which isn't hosted on GitHub, though it has mirrors. Most corporate intranets, SourceForge, GitLab, Sourcehut, and probably most programmers' laptops have Git repositories that do not push to GitHub.
Those people won't be vulnerable to this attack, since this attack is only useful in supply chain attacks. The people vulnerable to this would be maintainers of open source repos who could end up approving a malicious PR.
You can certainly launch supply-chain attacks via SourceForge or GitLab; indeed, probably the most famous open-source supply-chain attack in history was carried out by SourceForge's former owners.
