I think it's most likely still going to be sending money "via iMessage" (or more precisely: via a book transfer between two Green Dot Visa prepaid card accounts, addressed by an iCloud address and initiated over iMessage in UX terms).
Or do they now allow it between two non-Apple-Cash debit cards? That would indeed require some network to work across banks.
https://www.macrumors.com/2024/06/11/tap-to-cash-ios-18-pay-...