I wouldn’t say that’s particularly surprising. Most people in Lemmy and similar platforms have been here since the mass exodus from Reddit, or are programmers themselves. These groups are usually more privacy-minded, and see this as a significant privacy issue. This doesn’t really necessarily mean it’s an echo chamber though, I’ve seen a lot of people talk about how they use and like Windows, and I think the reason why they downvoted your comment (making an assumption here, I don’t see downvotes in my instance) is because it seems to be completely unprompted by anything or anyone, and a bit abrasive.

What does that have to do with echo chambers, exactly?

(Do note I’m not an astrophysicist, so this may be a bit wrong, but I think the main part of it is right.) Not exactly. Everything in the universe is constantly drifting away from everything else. The reason it is pretty much only visible at the scale of galactic clusters is that literally every force in the universe overpowers this expansion, unless the distances between the objects are truly absurd, in the range of millions or billions of light years.