> For beta user, back when Monzo wasn't yet a bank account, the only to put money on it was to pay by card.

This is incorrect - it was possible for a long time to top-up via bank transfer. Certainly during beta. It simply required using a shared sort code/account number with a specific numeric reference which tied the payment to your account.

It wasn't as instant as it is now (with Faster Payments), but it was still definitely possible.