alt Hacker NewsGitHub == git, which is free. You can clone the repo and push it to wherever. No comment on the rest. I am just pointing out that using GitHub for source code doesn't mean mean that code can't be easily forked or used elsewhere. I suspect GitHub is for convenience since the majority of folks using git use it.
The git part of github is a very small part of it, they are using Github issues etc which are not exportable/cross-platform.
So yes, you can git clone the repo and get the HTML, but if you want all the other stuff (the "github based workflows") then you have to use github.