In kaibee's first paragraph, there's a missing question: Is it collusion to pay someone to set rates accordingly by using information and decision-making processes you yourself could not have gotten on your own?
I think this is the key. E.g., looking at Zillow could tell a given landlord that they need to charge more rent for a given unit. But looking at Zillow couldn't tell that same landlord to, say, keep that unit vacant for months in the hopes there is a multi-landlord unit-vacancy plan to increase prices across a city. And IIRC, these kinds of decisions have been made by a lot of landlords in a lot of these markets.
In fact, if I'm a landlord, I'm not going to take that risk unless I've got a good reason to believe that a) the software I'm relying on is used by the vast majority of other landlords, and b) the software is advising and strongly incentivizing everyone else to do the same thing. (And possibly that there are penalties if the other landlords don't make that move.)