It’s one way to find relays. 

There are multiple technical solutions to chat with folks with whom you do not share a relay with. 

And yes, there is an unhappy path where you might not be able to find someone’s note. 

 I'd love to know what some of those technical solutions are! 

 Search for gossip, outbox, relay hints. A lot of discussion round this parts recently 😅.

A more basic hack is broadcast the parent note to your relays when replying.