There’s not much to reinvent. They could have setup & hosted an XMPP server with MUCs & microblogged posts on whatever ActivityPub option they chose (or similar softwares).

The point isn't to go back to the 90s, it's to bring the 90s into today.

How is creating a Mastodon account equates to "going back to the 90s"?

Instead of there being one central authority where you create your "Mastodon account" that is responsible for authentication and deciding who gets to view and post what, there are thousands of independent 'authorities' each deciding who they want to let on and how they want to do things and what subset of Mastodon they want their users to see. Much like Usenet and IRC

Modern XEPs have brought XMPP to today. ActivityPub is fairly new too.