Just note the license for Mattermost is AGPL or you pay them to free the code, only the compiled bits are MIT.
AGPL is also a free software license
Sure, but some employers forbid such licenses, AGPL is way more strict on making you share code than the GPL itself is. There's often libraries licensed under AGPL which could force a company to release all their backend code, which would be bad.
But you are probably not integrating this into your own code, so it probably should not matter?