Will OpenSSL reject properly implemented features contributed by third parties, because you sell paid features on top of OpenSSL and said contributions happen to make your product redundant?
That's the difference. Mattermost won't ever accept contributions, that reimplement features from their paid product. If you want to use such features, you would have to fork, and forced to keep the fork public, since it is AGPL.
https://github.com/mattermost