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.