It helps that IRC is extremely simple. Most servers you can connect to and chat using telnet if you're so inclined.
As others have pointed out, there's also a pretty decent range of very usable "daily driver" clients these days (Element, weechat, FluffyChat, nheko, Quaternion, NeoChat all spring to mind). 2nd generation servers are also progressing well (Dendrite in Go & Conduit in Rust).
That linked message has a lot of good information though.
> this is specific to a homeserver implementation
which suggests that it's not documenting a Matrix protocol. Unless Matrix has no standardized client protocol? Are all clients going to be homeserver-specific?
Even so I'd hope there's some document that says "Matrix standardizes server-server communication, client-server communication is out of the scope of this standard and is left to individual implementations" or something like that.