That's not decentralized, that's just open source.
> Being able to point your Facebook Messenger client at a different server would be a big step.
Interestingly, you used to be able to do the inverse: point your XMPP client at Facebook Messenger. I'm pretty sure they never supported federation, but you could connect clients to multiple servers and have a relatively seamless experience from a single client.
The "network" part is arguable, but IRC does not require any pre-ordained nodes, which matches the author's definition.
It's a small part of full decentralization, but an important one. Being able to point your Facebook Messenger client at a different server would be a big step.