No, it's a "grant to Mattermost and to recipients of software distributed by Mattermost a perpetual, worldwide, non-exclusive, no-charge, royalty-free, irrevocable copyright license to reproduce, prepare derivative works of, publicly display, publicly perform, sublicense, and distribute your contributions and such derivative works."

https://www.mattermost.org/mattermost-contributor-agreement/

https://spinpunch.wufoo.com/forms/r3z8zqm0ew6pcc