The fediverse is (generally) an open federation, not a closed one like you're describing. There is no manually-curated list of instances that you federate with.
I would expect "Open Federation" does not mean you need to talk to every instance out there directly. But that it works like a web where messages are routed around. I could be wrong. But I would expect the "servers_i_talk_to" array is what the instances output at the "peers" endpoint:
There's not really any routing, but you don't need to send posts to every instance, just every instance that has users following your instance's users.
https://fosstodon.org/api/v1/instance/peers