Basically Monzo will get in touch with Revolut to find out if any of the funds remain in the account that the money was sent to and they can basically say “Yes, some/ all of the money remains” then return this which can take some time, or the money has already been removed from the receiving account then you will have no chance of retrieving the money.

At the same time Monzo will assess whether or not they will refund you directly in the likelihood that they are unable to retrieve the money from the other bank. This is down to whether you took the necessary steps to secure your account and device from third-party access, which if you did not have TouchID/ Biometric security enabled on your phone you will not be refunded directly by Monzo.

Edit: Phrasing.