You must be part of an exceptionally small minority then. I, too, have known about Mastodon since it's inception, and this was not news to me. As another commenter said, a debate during that time was that users were beaten over the head with too much discussion of federation and instances.

This is always how it's worked, and it's nearly impossible to join any instance without following & interacting with people from other instances on a daily basis. It's like the one thing about Mastodon you can't escape knowing about even if you actively try.

I don't use Mastodon, so perhaps that's why. It always seemed like a Twitter clone except with fewer people and, importantly for this particular discussion, where you can't interact with anyone.

Guess I was mistaken, but that's why I never bothered to learn anything further about it.

Not sure how much clearer it gets.

How can you not know, since it's clearly "decentralized" in every claim made about its service. That's the textbook definition of decentralized.