If these landlords were colluding with eachother to punish defectors that undercut them, that would be a cartel. If they are all reacting to a shortage by raising their prices, that just makes them... Market agents.
It’s just in a broken market - there isn’t enough supply of housing, so prices are going up. And because everybody needs housing, it feels icky to many people (I’m one of them, but my solution would be relax zoning and allow more housing).
Unless using this system requires participants to set a price, I’m not sure it meets the definition of price fixing. A landlord can still underprice to guarantee a tenant quickly. Or overprice in the hopes they catch a low-info, high-income tenant.
OPEC works because they all move together. One of them raising or lowering production alone doesn't do much; all of them doing it at similar times does.
Same with RealPage. By having nearly everyone use it, nearly everyone's pricing moves in a coordinated fashion, and they collectively reduce the chances of downward price pressure.
Landlords don't always advertise, and they also negotiate rents. Knowing the actual market price isn't easy (and sometimes not possible), because that information isn't publicly available and certainly isn't real-time.
RealPage is getting private market information from landlords, and using it to coordinate rent prices, including coordinating actions to remove supply from the market to artificially increase prices. This removes competition from an otherwise competitive market. This is collusion via a middleman.