It's mostly static files, if it was a modern SPA with APIs and such, it would probably crash having to fetch the same data for what is quite literally a static site.
I think I've mentioned this before, but nothing about SPAs require that level of bloat. My personal site (https://chadnauseam.com/) uses React and SPA-type features like preloading internal pages so they load instantly when you click a link, but almost all of it works fine with js disabled. It used to get a perfect score on lighthouse too but it doesn't anymore :(