The whole idea of anti-monopoly laws is to prevent price fixing by market, and there are numerous (illegal) ways to do that: being a monopoly and using that position to set prices that are not representative of market value (both buying and selling. Iirc antitrust in the us started as a result of standard oil setting the price oil would be bought from suppliers).
Another illegal option is collusion. That is a group of competitors get together and set a price that they will all use, again independent of actual
Market value, just because the colluders have sufficient control of a market that when they set a price people have no choice but to pay it. This is super effective when the market it not fundamentally “free” like housing, gas, power, healthcare, etc.
What these companies are doing is providing a tool to launder the collusion between competitors in a market, by having every “competitor” in the market get an “algorithmic” price that is fundamentally tied to the “algorithmic” price they provide every other “competitor”.
If these landlords got together in a room and decided the prices as is happening here, it would be more or less immediately subjected to scrutiny. By doing the same thing via a third party and calling it an “algorithm” it is somehow not subject to the same restrictions.