but is way more than relay discovery

relay discovery can also be "find the relays and read/write everything, everywhere"

outbox is much more descriptive of what's happening and it's also less intuitive, which is a good thing, because someone sees "relay discovery" and could assume that they know what you're talking about without having to dig deeper, whereas both gossip and outbox have the "wtf are you talking about" which forces you to ask or remain willingly ignorant