Missing the point, which is that the lookup doesn't need to function or be available for anyone but the person composing the message at the time they compose it. It can be entirely private, entirely public, or a mix. What gets transmitted is the pubkey. Optionally the relay can provide a mapping, which may be globally consistent or entirely based in some private mapping.

Users just need a catalog which includes the keys of people they care about. The relays can backfill this information. You have little reason to know mappings for users whose messages you're not seeing, and if the messages are not censored the mappings won't be either.