I should have written more clearly: Github's pull-request workflow is regarded by some (obviously including myself) to be the "extend" part of an "embrace, extend, extinguish" play. In other words, 1) embrace Git, 2) extend it using a Github-only feature and encouraging an entire generation of developers to think that you can't use Git without pull-requests, 3) making it some sort of proprietary product.
IIRC Sourcehut was Drew's way to develop a Git hosting solution that integrated the Linux kernel project's workflow. The idea being that this is a completely open-source project that uses open standards, i.e. email, rather than proprietary things like pull-request.
He wrote a great article on the email-driven workflow: https://drewdevault.com/2018/07/02/Email-driven-git.html
You're right, though, Github has done a service to the open-source community, and nobody forces you to use pull-requests.