kpw
The NGO is a decoy organization with exactly the same people (minus one) as the VC funded startup. Go look at the “core spec team” and find out which organization they belong to.
Your information on XMPP seems to be quite outdated. File transfer in XMPP is now mostly done by uploading the file via HTTP and sending the URL. Audio calls are done using WebRTC and work two ways.
“protocol extensions” (aka: incompatible)
Reality shows that implementations can very well implement the same extensions. If you don’t use extremely outdated clients you will find they do have compatible file transfer and A/V calls. ActivityPub works the same way.
Meanwhile Matrix Ltd. cooks up a completely new, incompatible protocol instead of building upon existing internet standards.