"Subject to the exceptions" conflicts with the "no exceptions" wording in the GPL licenses, so I don't even see how any of this constitutes a valid license
https://github.com/mattermost/mattermost/blob/master/LICENSE...
I'm not sure that they actually granted a GPL license at all though. I could see this document being read as an advertisement that one might be for sale instead of a grant...
(Not a lawyer)