"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
I read the exceptions as a grant of additional rights to mattermost's copyright (but not to third parties), I don't think that conflicts.
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...
https://github.com/mattermost/mattermost/blob/master/LICENSE...
https://www.gnu.org/licenses/agpl-3.0.en.html