Maybe it was using technology similar to magic wormhole.

https://www.youtube.com/watch?v=3G3_Xa1GBTs

https://github.com/magic-wormhole/magic-wormhole

Sure, strictly speaking, of course there were accounts as in "database records". However, users gave no details about themselves, and haven't signed up for anything. No long-term credentials, just a rendezvous point.

You used to just open two Firefoxes, start sync setup on the one, copy (or scan) a code to another - boom, done, they're talking to each other.

It depends how you define an account. Is an account an email? Is an account an id in a database? An IP? You can't sync anything without an "account"