I agree with your point about the difference between the lemmy and mastodon websites; lemmy could do a significantly better job attracting more users if it changed the welcome page to make the more familiar features clearer.
At the end of the day, federation isn't the important part here IMO. Lemmy is still a social media platform, and social media rides or dies on engagement. Without users, there isn't engagement. I know that tiny communities can exist for years just fine online, and that lemmy did so with a very small user base. But, if people on here seriously want a true alternative to other social media, that alternative needs to show that it has enough engagement to keep people coming back.
To me it's much more important to show prospective users that the tools for engaging with the sited platform are familiar and easy to use, than it is to dive into the details of how federation works. The average user simply won't care about federation unless it directly affects them.
And frankly, those old internet forums went out of style for a good reason. It was annoying to have 10 accounts to keep track of on 10 different social media sites. I don't think anyone today would say that signing up or distinct forums that fo us on topics you care about is a true alternative to big social media like reddit. The way lemmy talks about itself right now makes it seem like you're back on the old forum system, when the whole point of federation is exactly the opposite of that! This FIXES the "you need 10 accounts" problem for the most part. At the end of the day, people want to know that a thing is easy to use and understand, and that other people with similar interests are using it.