The reason I skipped over this was because the author seemed to be pushing an agenda elsewhere in the thread ("Matrix is like XMPP but with a heavier server": https://news.ycombinator.com/item?id=26279816, and "Matrix has one reference client/server with really good features but hardware requirements that place it out of ordinary folks' reach.": https://news.ycombinator.com/item?id=26279780) which made me wonder whether answering all their posts was going to be a good use of time. In practice, the thread hasn't exploded into a XMPP v. Matrix holy war, so it looks like my concerns were ill-founded.
To actually answer the question from the post in question:
> Is there a working group talking about vocabulary or interop with other federated networks (ActivityPub/XMPP)?
No, there isn't a formal WG - instead there are a set of informal conversations between the 3 projects. I lurk on xsf@muc.xmpp.org and standards@xmpp.org in case interop discussions come up; I also hang around various cross-project rooms where Gargron and Christopher Allen Lemmer and others are active on the AP side. As a result, cross-project initiatives sometimes pop up - for instance Matrix supported Mastodon in adding E2EE at https://github.com/tootsuite/mastodon/pull/13820, and we've also tried to work with XMPP on using Olm for E2EE as an alternative to libsignalprotocol in OMEMO.
In terms of content vocabulary specifically: we researched JSON-LD a bunch when working on MSC1767 (back when it was MSC1225) - the notes predate us using GFM for MSCs and are archived at https://docs.google.com/document/d/1m4VTRqclB3JEMZBjbr4t5cvI...
In practice, I'm not convinced there's a huge amount of value of standardising on the same vocabulary though - whether that's MIME, JSON-LD, RDF-flavoured JSON-LD, IPLD, rich XMPP stanzas, etc - as long as there's a way to map between them. Given you need to map the protocol layer in general when bridging protocols, why not map the content format too? That said, if folks have solved the problem elsewhere satisfactorily, then we'd have no problem with piggybacking on it on Matrix (just as others have piggybacked on Matrix's E2EE).
In terms of the parent's comment:
> I'll hazard a guess: New Vector LLC sees it is in their financial interest that they own the only functional implementations of the Matrix protocol and what it can interact with, so there's no interest in federation with the wider Fediverse and other protocols.
No, my priority is trying to make the wider Matrix ecosystem as diverse and healthy as possible, including bridging nicely other protocols into Matrix. The more different projects building on Matrix (e.g. hummingbard) the more potential in the network and protocol for everyone involved. A rising tide lifts all ships, as it were. My dream for Matrix is to become the realtime communication layer of the open Web, and for there to be as many different parties building on it as do the open Web today. If we pull that off, then Element (formerly known as New Vector) would hopefully benefit from it as much as everyone else - much as 3Com benefited from creating Ethernet and bootstrapping that industry alongside DEC, Intel, Xerox and others.
But it would be UTTERLY STUPID for Element to privilege itself over other contributors to Matrix, and would effectively self-sabotage the open ecosystem, thus killing the main thing which could make Element successful in the long term. Hence the huge amount of effort and $ that we put into setting up The Matrix.org Foundation (https://matrix.org/foundation) as a neutral independent guardian of the protocol, and protect it from anyone commercially building on Matrix... including Element/New Vector.
Sorry you think we suck though :)