This does highlight one of the perennial issues I’ve found deploying Discourse among even technical user-bases - they fundamentally misunderstand the distinction between Categories and Groups. Every. Time.
This seems to be caused by people’s mental model of what a ‘group’ is - they have learned this from the way Facebook Groups, Google Groups and WhatsApp Groups work - the Group is the place where the discussion happens, and honestly trying to get people to learn this anew for Discourse has at times made me borderline murderous.