Thanks Ian. Unfortunately, not at liberty to discuss the usecase in detail but I will give you the scale of numbers to being with . Think 1 million users, each having 100 conversations each month and each conversation containing 100 messages.
I know you linked to a page which mentions mattermost can easily support 10s of thousands of users. What the FAQ doesn't do is appease me based on the numbers I have mentioned.
So, what say you? : )