I get the argument, but email is also very different to the kind of open-web network that the fediverse resides in. There are problems the fediverse faces which email doesn’t like discoverability. The emails either come to you or they don’t. With federated social media, you have to find the content you’re looking for first. Maybe you use a search engine, or somebody gives you a business card with their handle and instance, whatever. Then you have to figure out how to view those posts from your home instance if you want to actually interact in any way. There’s browser extensions and stuff which try to make this easier, but that’s another thing that has to be explained and set up, plus not everyone is visiting from a web browser with extension support, or a web browser at all for that matter.
It’s not fundamentally impossible to understand the fediverse, but there’s more of a barrier than email, which can be explained in a single sentence like “Your email provider gives you a unique address that anybody else can send emails to and vice versa.” I don’t think convincing ourselves that the fediverse is actually very simple is going to convince people outside the bubble that that’s true.
That’s a pretty normal part of the fediverse. Somebody from your instance has to be subbed to a community for your instance to start pulling in its posts. Anything that was posted while nobody was subbed doesn’t exist on your end.