The article explains what's needed to run something like Bluesky (ctrl-f "To be fully sovereign you need").

My understanding is that Eurosky aims to be a non-profit ran alternative, hosted in EU. It integrates with Bluesky seamlessly (Bsky users and EUSky users can interact) but would keep working if Bluesky was taken down. I believe it also gives Eurosky agency when it comes to moderation.

They are, it's just a second instance of a realy. For independance, resilience, moderation, and also probably minor technical choices.