Exactly. We need to get really obnoxious with the promotion

It's pretty likely they've heard of it as a niche option, as a forgettable factoid years ago.

Then they may also have learned it's a very small platform and something about decentralization.

if they went to sign up regardless, it seems likely they didn't end up picking an instance. (Possibly having left the page open, briefly looked up Mastodon instances and read a few sentences, then moved on to something else.)

Now that they wouldn't have had to pick an instance, they've long forgotten about it, maybe you could get them to try the normalized sign up process.

But anyone who knows anything about people knew from the start that Mastodon wasn't really even aiming for successful growth. Of course the instances were gonna be an obstacle.