It'd be interesting to see how that would work with federation. But conceptually implementing any kind of event that works across instances is a fascinating idea.

I think having matrix is useful for eventually letting them federate without needing a new account - just enable it on the server. Nextcloud chat won't have that advantage

That last sentence is a bit annoying. Why wouldn't I trust anarchists with data? Did they just pull up a definition of kolektiva without actually understanding what anarchism is? The last sentence really implies they're using the incorrect but common definition of anarchism meaning "no rules" instead of the actual definition "no unjust hierarchies"

To clarify, that's for them hosting the service for you, and is intended for enterprises mainly. You can self host element, since it's open source, while only paying your own server costs, or just use an existing instance like on https://app.element.io