This is very common in the hospitality industry. There is nothing illegal about it. We did this frequently with our hotel on the booking sites and it is perfectly legal. When you see 1 room left on the hotels resale sites, it just mean I have fifteen of that type and will add another to the inventory when someone books it.
This is a lie. "out of service" is how you don't have to report a room as inventory. A room without a working toilet or P-Tech air conditioner can not be rented and is out of service. Also if you are doing a refurb and the rooms are under construction of some sort they are not in your inventory. There are many other reasons, but one of them is if you do not have the staffing to service the rooms. It does not matter if you don't have enough employees to service the rooms, or your choose not to staff employees you have. I owned a hotel three miles from WDW for many years. Occupancy is a shell game. The quote from WDW you put out has the word "temporarily" in it. That would be a room that would be a room taken offline for a short period due to a housekeeping or another problem a guest or a staff member discovered when getting the room ready. An "out of service room" is never counted in inventory.