Just to add another data point. I made a reservation for FHR just before midnight on 12/31 . I selected pay now non refundable rate - showing only resort fee due to be paid at the hotel. I immediately received 2 emails- one from Amex confirming the reservation and another one from the hotel via Amex travel - hotel welcome letter. Though I selected non refundable rate- the hotel cancellation policy allows for cancellation or changes with a small fee up to 2 weeks prior to stay.
The travel credit posted today dated 12/31- credited for 2024.
The process worked the same for the same hotel for me in 2023.