I think the original analogy works better.

If an EU country goes rogue, other EU nations can't just isolate it and bar it's citizens from entry. There is no expulsion from the EU AFAIK. But Lemmy instances can block another instance fairly easily and unilaterally - like how nations can refuse visa to citizens of a rogue nation. And Lemmy instanced are expected to federate with most other instances, just like countries are expected to grant visas to most other countries - unlike joining the EU, which is a whole big process and all EU members have to agree (there are no vetoes in Lemmy federation).

But most importantly, the EU members are required to act as one in many circumstances - most laws apply across all EU members, EU negotiates trade deals as a block, etc. That is not true for Lemmy instances. Each is completely independent and makes its own laws - and must only comply with some very loose principles (which boil down to "don't be a total jerk") to not be isolated from other instances. This is much closer to the kind of independence countries have, than EU members.