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.
But this never existed, nor is the author claiming that. AIM, ICQ, etc required you to create an account on their own service. XMPP might be used, but I don't think any major service besides Google Talk supported federation. What the author had was a single client that happened to connect to multiple services, but you can do the same nowadays (e.g. using Pidgin and its plugins).
(As an aside, it's refreshing to see how BS-free the announcement that GTalk supported federation was: http://googletalk.blogspot.com/2006/01/xmpp-federation.html)
>> I loved the IM era and it's a huge shame it's over.
> I find that Discord is bringing it back.