Haha. "Oh yes, Slack, thank you for doing our marketing for us."
I can't tell if that's pompous or genius. Or just really smooth riding of coattails. Or all three.
I've tried Mattermost and while it doesn't feel completely ready for primetime it's got promise. An open-source Slack is something I'm sure a lot of people will be interested in, but it's yet to be seen how profitable their choice of revenue model will be.