According to RichardR:
I don’t know technical details, but on a theoretical level, it might be possible to construct an NFC tag which provides cryptographically signed transaction info and beam that across. The issue with tap to activate card is that the NFC interface needs to run the Mastercard Debit app on the card, and that is not possible with Core NFC. But Monzo app-to-Monzo app might be able to use tags since the app is more flexible than what’s running on an EMV certified card.
I’m not a developer, so this is a bit of wild speculation!