Well, yes, if you continuously adjust your prices to match your competitors, rather than basing your changes on other factors (changes in maintenance expenses, inflation, property values as a whole, changes in area demographics like crime rate, school quality, walkability, availability of services, etc), and you do this at the same time they do, then that can be an indicator to the law that you're engaging in price fixing.

But it's not the only indicator, and likely is insufficient on its own if you don't have significant enough share of the local market for changes in the prices you make to ripple outwards to smaller players. It's hard enough to get them to go after the big players, so they won't go after you.

Usually price fixing requires multiple indicators. The law doesn't have to prove that Bob is talking with Alice and prove that they're discussing how to raise rents with each other. Prosecutors just need to demonstrate that Bob and Alice are both doing the same things, around the same time, which contribute towards artificial increases.

As players get larger, their policies and procedures have greater impact over an area. Their practices get more scrutiny because of this greater impact. That's one (of many) reasons we see large companies spread themselves to multiple cities. While nobody bothers when someone who rents a dozen units uses algorithms to set their prices, they start to notice more when it's thousands of units all concentrated in a single area.

When you get to a position where 4-5 companies can significantly inflate prices for a city with millions of people, how they behave is important.

There are other tricks they use beyond third party algorithmic engagement.

Consider this:

If only 30% of the rental market engage in a laddered renewal scheme where they push renters towards always renewing within the same 2 month period of the year (via 10 or 14 month renewals that are more financially viable than 12 month renewals, or are simply the only option for renewal until the renter is within that two month period) and also add significant lease penalties for breaking out of that schedule, it takes less than a decade to reach a point where 90+% of rentals are renewing within the same two months every year.

And what does that do? It artificially constricts supply (nobody releases their lease until they've found a new apartment, so many units are not displayed as available or need to be accepted sight-unseen) and artificially inflates demand (everybody is looking during this same time period).

The result is artificially increased rent. Which has a side effect of inflating bubbles in the housing market. That may _seem_ good for property owners, as they can sell their houses for more. For a time, until the next pop that everyone blames on lenders with dodgy loan acceptance criteria or other criteria. Until then, homeowners pay inflated property taxes. They might overextend their mortgage with the new perceived value of their home that may eventually drop, etc.

The point is: rental price fixing is real and deliberate, large players with greater impact know better but do it anyway, and this is not the only trick they use. It's just the one they've currently stepped far enough over the line on to get caught at.