That is incorrect. I have had several clients where I have been able to book them in different staterooms on the same sailing. Royals systems, unlike other cruise lines do not pick it up.
Here's one of the situations where this has been done. Family of 4 booking a cruise 2 years out. Won't know until closer to the date if 1 child can go or if both can go. If all 4 go they get 2 rooms, if 3 go then they get 1 room. The one room with the three of them has all the same people that are booked in the 2 rooms. One set oif bookings would be cancelled before final payment