Likely the agent that answered is the Asiana Reservations, which is open on holidays. However for Asiana mileage awards, you would also need the Asiana Club office to be open, which they are not open on weekends and holidays. The Asiana Club office also has limited hours, 08:30~12:00, 13:00~17:30 PST.

Even for the Korea Reservations and Asiana Club, they are not 24 hours. They are 07:00 ~ 22:00 KST (with Asiana Club likely ending at 18:00, along with Ticketing)

There's a segregation of responsibilities, the Reservations agent creates the booking, however they would need to transfer you to a Club agent to process mileage payment and ticketing.

They may have turned off the rerouting to avoid their call centres from being flooded with reduced staffing levels given the current COVID situation.