I think you've proven the point. Exchanging keys on a bike forum first or chess club means discovery is happening on external forums, not on Pubky. That's outsourcing the difficult part of the problem a social network has. Nostr discovery is plural and interchangeable from day 1. And it's honest about it.

The external forums [venues] are indeed an onramp, and means to establish the first few peers [followers/friends]. But thereafter, a Nexus server isn't required in order for users to expand their network, and crawl results [index files] can be stored locally. Relays/intermediaries are not foundational to bidirectional communication on Pubky, because peers can push and pull data directly between one another. Coupled with Kademlia (see: PKARR) this confers true decentralised mesh properties. Nostr exists inside an airlock, and uses authorities to relay data.