General question. Is this gitesque approach useful for online courses, code distributions, forums etc. instead of let's say a phpBB?
Code contributions are quite obviously simpler with GitHub. Hell, phpBB is bad for almost anything
Distributions... GitHub used to be good at that; not any more, as far as I can tell — you may be better off hosting the files on AWS and just linking to those from github pages/wiki.
Git as a forum isn't that good; on the other hand, I can see extensive discussions in github issues, see e.g. https://github.com/Reactive-Extensions/Rx.NET/issues/19
For online courses, I cannot see a point. Having said that, no MOOC I saw so far (and I've finished a dozen) had a good forum. Slack groups work well for that, sites... meh.