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.
It's time to stop using https, instead let's use a new protocol owned by Facebook to visit web pages.
It's time to stop using IRC, instead let's use Slack.
It's time to stop using email, instead let's use a proprietary messaging app.
OR let's not commit everything to corporate-controlled protocols and platforms, and keep SMS, RSS, HTTPS, email, and so on. Please.