clicking around GitHub and checking network panel, it seems to load plenty of server rendered HTML. Some views seem to use React within the page, but it doesn't appear to be a React SPA.

Maybe I should have said pre-react. I don't know what GitHub did specifically, but several years ago it used to be reasonably fast and relatively pleasant to use. It regressed a lot over the last few years, seemingly correlated with attempts at interactive features.

It used to be jQuery + PJAX

More recently - and still - it was web components. React is gradually creeping in.

Github's Primer design system was wonderful when it was a pure CSS system that could be used with any framework. Sadly, M$ killed that and made the new Primer design system shitty-React consumption only.