The Mattermost preview screenshot is an odd choice - have a read of the conversation.
The text of the screenshot was actually written in Slack, and the goal of the screenshot was to demonstrate the Slack Import feature to GitLab Mattermost.
On the GitLab Mattermost page (https://gitlab.com/gitlab-org/gitlab-mattermost) there was a subtitle describing this, but we could have made it more clear in the screenshot itself.
Keep reading.