I argued for multiple relay hints a long long time ago but it didn't get any traction, got pushback on being too much overhead instead, and pushback on the fact that these things rot over time.

The NIP-02 relay hints are 3rd party.  Somebody's follow list claims somebody else uses that relay, but do they really?  That is why I don't use them for authorative data.

The authorative data is NIP-65 kind 10002 (and new 10050 for DMs).  That is meant to be the real deal.

But in case somebody didn't publish what relays they use, there are fallbacks:
1) The contents of kind-3 (although not documented in NIP-02), since some clients put them in there for self-configuration
2) relay hints in other people's kind-3 (NIP-02), but these are 3rd party
3) NIP-05 nostr.json may have relays, but nostr.json is unsigned!
4) Maybe you saw their events on a relay before, so you might remember that as a possibility
5) Any 'p' tag can have a relay hint (again 3rd party, plus they rot, but if you have nothing else....)

Gossip keeps all these relationships and scores them, and it has been a long road of tweaks to that system to get where it is today.  If all the other clients did the outbox model, I could throw all that stuff away and just use NIP-65 and be done.