Early adopters of almost anything tend to be niche. These Threadiverse sites are looking to pick up where pseudo-message boards like Digg and Reddit left off without being extremist havens like Voat and other bullshit. So let's look at who the early adopters of those sites were. Because... They're not that dissimilar to the demographics that you're describing. Reddit didn't start out as the kind of place that just anyone went to. It tended to be tech heads in their mid-twenties or older, gamers, and chronically online people. They tended heavily to be male. And there tended to be some... Really unfortunate widely-shared opinions.
As Reddit grew, it changed. But it took time. It took there being content on Reddit to appeal to a wider set of people. And that's going to be the case here. It needs to reach a first sustainable mass where enough content is being created to engage and keep the users who first joined it. But that userbase is going to be rather similar. There are always going to be subgroups that are different, but for the most part, the same kinds of people are going to be the early adopters. Creating a breadth of content that will appeal to more and attract a wider variety of users over time will help people feel more comfortable with it.
And, yes. The Fediverse is kind of weird to most people. I was in an argument the other day where someone was insisting that saying you saw something on Limmy or KBin was wrong, you saw it on the Fediverse, and could everyone just stop being wrong please. That kind of pedantic culture is only going to make adoption even slower than it already is. Because most people, they like to go to a site and create a login to look at that content. The Fediverse isn't really that complicated, but it takes a little jump in how you think about websites to go from something like Twitter or Facebook to something like Lemmy or Mastadon. But people were kind of confused about the leap from message boards to social media like MySpace and Facebook as first too. They came around. It took time. It took exposure to the content. It took people using it and sharing it.
So, yes. The Fediverse is mostly a monoculture right now, focused on the people most likely to make the most of out it: Tech heads with some time on their hands for hobbies. The kind of people who either might make their own Fediverse instance or who would know the people that would. Those tech heads aren't exclusively Linux users, they're not exclusively over the age of thirty, and tech heads aren't exclusively the user base, but yes, we're going to start out seeing an imbalance. That's normal. That's to be expected. What's going to be concerning is if five years from now we have the same or a worse imbalance. That will mean that the Fediverse is stagnant or shrinking instead of growing. That will be a time to rethink some strategies for sure. But for right now, all we can do is be active, share the site with other people, and try to get it to spread to more diverse demographics.