To me that feels like somethign that would be high value to landlords with small numbers of units though. Once you are one of the big dog with hundreds or thousands of units in the area (which my impression is largely the companies using the data), it feels like your own data likely gets you 90-95% of the way there anyways.
So does it suddenly become OK if RealPage pivots to only using a particular landlord's data for that landlord? Then it is largely the same rent-setting without it looking like collusion.
There's always a benefit to collusion, otherwise companies wouldn't keep trying it and/or looking for loopholes like letting an algorithm do the collusion.
But yes, on a legal level, if the Realpage algorithm was looking at solely one particular landlord's private data (along with public data for all the other rental properties) to form recommendations for that landlord, then it wouldn't be collusion. If one landlord is so large as to be able to drive the market on their own though, then there's other monopoly/anti-trust laws that should come into play.
The idea of competition is that if there's meaningful competition, then any one company can't unreasonably raise prices, as people will switch to using the competitor. If competition becomes basically non-existent, either through collusion or just one company basically becoming a monopoly, then something is wrong and needs to be fixed.