You won't know until you call, but as is the case with any other Disney discount, you can "apply" a discount if the room is available to be booked with the promotion
as a new reservation. Rooms that are selling well are often not available to be booked with the discount. For example, Poly is usually very popular and it's often difficult to find availability with discount, as are the LM rooms at AOA.
It's like when you go looking at what rooms are available with a public discount (like the
Disney+) there are many rooms available to be booked at rack rate, but there is a much more limited selection available with the discount. If you had a room booked, you can see if it appears with the discount when you go to modify it, it doesn't always show as being able to be modified with a discount.