Yep, but unless you train your team up on using Mou or just reading raw markdown, you can't actually browse or publish to the wiki ("use or mutate state"), and again the usability is horrible so nobody will do it.

Usability is the huge selling point of Github, so any replacement would need to be as slick

It's Gollum. So if you install the Gollum gem and tell it to serve the directory, it'll serve it up just fine.