I sent them. But this is the reply I got.
As it is a prepaid, nonrefundable rate, we would require the full payment via credit card to book the room as the option to pay with points is not valid for a prepaid rate. Once you check in, we can credit the full amount of points to your reservation which will pay for the room, as you need to be present at the hotel for us to credit the points. The amount paid by credit card, can be used for any incidentals or extras and the amount that remains we will refund at check out.
On checking further for double confirmation, they replied below.
You can indeed book the non-refundable Advanced Saver Rate through the app, paying the full amount with your credit card.
Once you are at the hotel, we will credit the full amount of points to cover the room. Once you check out, any remaining funds from your credit card payment will be refunded to you, taking into account that if you have any extras during your stay, this will be covered by the amount you have already paid.