Mattermost team here. GitLab Mattermost is a binary, and GitLab community wanted the MIT license to align with GitLab's model, so that's the "commercial" license for GitLab.

One note to add is that the standard version of Mattermost is Apache-AGPL similar to MongoDB. So any organization comfortable with MongoDB should be fine with Mattermost.

MongoDB and Mattermost have Apache for drivers, configuration and administration so you can use and deploy the binaries without restriction. If modifications are made to the core product, those are required to be shared with the open source community.

From what I can tell, the dev team of Mattermost is essentially releasing two versions: you can get the original source under the AGPL, or a compiled version under MIT (called Gitlab Mattermost). Since they own the copyright, they don't need to abide by the AGPL when producing the compiled version.

An interesting side effect is that if you were able to decompile that version, you could in theory have a MIT license to the Mattermost source, avoiding the AGPL.

I guess they are saying all of the Gitlab code is MIT and all of the Mattermost code is AGPL. That's perfectly permissable by both MIT and AGPL licenses.

In practical terms, you can take the Gitlab/Mattermost distribution and modify the Gitlab code without having to release your changes. But if you modify the Mattermost code, you need to release the changes you made to the Mattermost code.