"*opolies" =/= price fixing/collusion, which is what the FBI is investigating them for. Even if you don't have a monopoly but you agree to fix prices with a competitor, it's still illegal.
The hair splitting meme around "monopoly" and effective monopoly power (and the common meaning of these terms) is just ugly.
You conveniently left out the part before: "at least 21 large landlords and institutional investors [...]". Wikipedia says the definition of oligopoly is:
"An oligopoly (from Ancient Greek ὀλίγος (olígos) 'few', and πωλέω (pōléō) 'to sell') is a market in which control over an industry lies in the hands of a few large sellers who own a dominant share of the market."
"at least 21" is not "a few" by any stretch of the imagination.
As far as the rental.market being an oligopoly, there's an argument to be made that the existence of things like RealPage makes large sections of the market act as it would with an oligopoly, even if it doesn't quite meet the definition.
That is an interesting conversation to have, separate from the illegal price fixing.