The article talks about how chat used to be a protocol, so you could talk between chat clients.

As though you could log into discord and start a conversation with someone who was only using facebook messenger, or snapchat, or twitter, or whatsapp or instagram or any other service where messaging was possible.

Discord in reality represents a different way of doing things. You can chat with other discord users. That's it.

It's a decent system when the userbase is big and the company is benevolent, but it's a different thing than everyone who does "messaging" agreeing to let messages from any service go to any other service.