The bluesky account jack@jglypt.net states that Nostr requires some sort of centralization because all users would need to have at least one relay in common so they can see all other posts on the network.

You tell us this is wrong, because "we have the Outbox model".

I disagree. Because the Outbox only guarantees that you are able to find all notes related to some note you already see. There can be notes on relays, you are not connected with and you will never see.

Especially if you are searching for something, e.g. using tags.