Booking a concierge room is pretty much a guaranteed cabana. But not necessarily. There are non-concierge guests that book cabanas also. It just depends on how many concierge/platinum cruisers want one 120 days out, then gold members at 90 days, and silver at 90 days. If not very many people are interested in them, it's slightly possible for a new cruiser to get one when their window opens.
More likely, a first time cruiser who's not concierge, or a silver
Castaway Club member, can snag one if someone cancels a reservation prior to the cruise (yes it does happen). Or onboard join the wait list and see if someone gives one up once the cruise starts.