Who said anything about immutable ledger? I was talking about signing updates so it can be verified when relayed through untrusted third parties.
So if lemmy.world sends an update to lemmy.selfhosted.guy and then lemmy.anotherselfhosted.guy federates with lemmy.world, lemmy.world can then say “Hey, here’s my public key, lemmy.selfhosted.guy have a copy of my data and is willing to be delegated to, you can get the data from him” and then lemmy.world doesn’t have to distribute it’s changes to everyone itself, but maybe just update 5-10 hosts. And the number of relays will scale with the network.
And thanks to the data being signed, you can trust the updates from lemmy.selfhosted.guy being from lemmy.world and not been tampered with, keeping lemmy.world as the authority of the data, even if you don’t trust lemmy.selfhosted.guy.
Edit: This would also mean all the small self hosted instances will boost the network instead of threatening to overload it.