Since this is aimed at new users who still need to create an account, them already having an nsecbunker running isn't really reasonable. Like nostr:npub1xtscya34g58tk0z605fvr788k263gsu6cy9x0mhnm87echrgufzsevkk5s said in another reply, other clients and service specifically there to run bunkers are the ones who will provide the backend.
The library uses NIP-89 (application handlers) to find nsecbunkers that support this sort of flow and then allow the user to select between them when creating an account.