Not the author, but I wouldn't say it's just (or even primarily) Github's pull requests or "critical security advisory" that's the problem here. PyPI requiring 2FA for maintainers of popular software has had more real-world impact (one maintainer took down and recreated their project, erasing old releases), and Google calling for deanonymizing (doxxing) maintainers of open-source software is more terrifying.

I'd argue that the problem isn't that "software supply chains do not exist", but "you using a program or library without pay does not and should not mean the software's author is now responsible for fulfilling your use cases and paperwork requirements".