At first they tried to say that "we're a school" and then when the MM rep said they have an Education license, they admitted that they are not actually a school, but rather a consulting company that is gouging schools by overcharging for open source software.
A user that was following the letter of the license and has suddenly had their access to the software restricted without warning.
Open source software means people are entirely within their rights to sell it to others, perhaps creating value by providing the warranty that all licenses expressly disclaim.
And there are 3 things that you can do when in this situation:
1) Pay the fee, if that is what is required for it to continue to be easy for you to re-sell the software.
2) Fork the project, remove the restrictions, and maintain it yourself.
3) Stop using the software.
All of those are perfectly within the spirit of FOSS.
It's about rug pulling your users and cutting them off at the knees. I don't use mattermost but read the github thread in it's entirety.