The template assumes the group id is "mediawiki-skin-X". But for the skins at GitHub the group id is "mwgithubskin-X". It should be configurable.
Group id in translatewiki.net
How are we supposed to handle that with this template?
I do not think the template was written with that use case in mind. I did not try this, but from looking at the template I think it should work to supply whatever you want in the
author param (e.g. some comma-separated list of author's names). If you then leave the
username empty, no broken links to some non-existent user talk page will be generated.