I’m sure if enough people got in contact about using open source communication they would likely attempt it
You see the chicken-egg situation here, right?
You can have multiple channels. You can bridge. You can designate some spaces as reserved but unofficial. They do list a Matrix in the finer print, but not choosing it as primary is madness IMO since the option are certainly good enough & if you believe in the philosophy you will direct your community in this direction to inspire other folks to uptake & hopefully improve our freedom-respecting options. Instead you start at bifurcating a community along lines of those that want ethical software & privacy over those who are willing/able to give it up—which as you say is definitely ironic given the marketing buzzwords chosen like “self-hosted”, “respects your privacy”, “open source”.