Seems the solution, #[7] also implement it like that based on https://github.com/jb55/cln-nostr-zapper/blob/master/index.js
Discussion
from how I understand this code this only works if connections to the relays are open an unauthenticated.
Yeah, I think so, only based on tagged relay and only works for tagged public relay. Devs need some workaround to add "popular public relay" probably such that zap response can be more visible 😅