cultures neither coincide with countries
I think that's very reductionist. Countries clearly have a large influence on culture and culture often forms around countries as people in a country share borders and law and politics and all that.
Anyways, we can agree to disagree if you insist. I do think you're being slightly closed-minded in this case though, but it's not a big deal.
Just want to note that a user-blocked instance still influences your feed via voting. The only way to truly and entirely block another instance is to get your admin to defederate the instance.
But on the fediverse you can choose your admin or become your own admin, so that's still a lot better than reddit.