I think it's highly unlikely that they're a true monopoly. Housing is just too fragmented a market. There are too many people willing to defect, too many mom and pop shops, and of course you have other cities that people can move to if they just can't handle the prices.
Housing is inelastic enough that they don't need a 100% monopoly to engage in highly monopolistic practices. The article mentions that about 80% of rentals in Atlanta are using algorithmic pricing, and 80-90% of landlords are following their recommendations as is. If the algorithm is laundering their collusion onto 64-72% of the housing market, that's larger than plenty of monopolies that have been broken up in the past.
