They did mention Matrix - which seems to be the best and most obvious choice, if you're trying to optimize for open-source, e2ee, run-on-any-device, not-tied-to

They did mention Matrix - which seems to be the best and most obvious choice, if you're trying to optimize for open-source, e2ee, run-on-any-device, not-tied-to-phone, and other features. Going for worse tech solutions will just make the discussion for which app to download for your friend group slightly shorter.