To me a wiki should have an UI on top of editable/versioned files, otherwise there is not point.
I agree with the parent poster that in this case, the UI is github, thus it's more a github wiki than a git wiki.
Do the purists who object to this (in a "Linus Torvalds' definition of git kind of way") fundamentally believe that there should be no central "origin" of any kind in a repo?
Sure, the main attraction in a DVCS is that everyone has a complete copy of the code and can pull branches or integrate changes from anyone else into their work, but eventually these changes have to 'land' somewhere and for most things, that's GitHub/GitLab/whatever other hosting service with git support.
^ This is - for practical purposes - my definition of origin.