From the press release: "The complaint separately alleges that RealPage has unlawfully maintained its monopoly over commercial revenue management software for multi-family dwellings in the United States, in which RealPage commands approximately 80% market share."
That’s 80% of companies who are using revenue management software are using Realpage, not that 80% of rental units are using Realpage revenue management software
... yeah that's pretty much all of them minus some mom and pops who manage themselves. And from personal experience those mom and pops are selling out to management companies now so a whole neighborhood of single family rentals might be actually controlled by just one or two companies.