If all units are being rented there is no collusion as the market clearing price is being charged. Collusion requires taking supply off the market in order to raise prices.
It is not possible to raise prices without lowering supply if the seller is already trying to maximize profit. Sure the seller could be undercutting the market but that has nothing to do with collusion. Apartments rent in an auction, each fills for the most anyone will rent it for. In order to raise prices landlords need to increase the competition for each unit.
This is factually incorrect.