Correct, a cruise using a placeholder booking must be new.
If the cruise were in more than a few months, you could book the new cruise now, book a placeholder in November, and then check whether the cost using the placeholder would be less than the cost you paid. If so, you could cancel and rebook using the placeholder discount. There would be a small chance of losing your selected room during the rebooking, but if you just need a room that sleeps 5 and not a specific room, you could even book using the placeholder then cancel to be sure of getting a room that meets you needs.
However, the paid-in-full date for a December cruise will be October, so you would pay a penalty for cancelling which would probably negate any savings.
If cabins that sleep 5 are selling out and that's what you need, you'd be better off grabbing the cruise you want now, and applying the placeholder to another cruise in the next 2 year.