I think they meant that Google managed to limit the number of bidders for ad placement - they shut out other advertising groups -, so they could then charge what they want to those who need to advertise their business or perish, and take what they want from websites that publish ads as well (take a larger cut from the 'ad inventory' understood either as ad space or ads to be published). In this sense the linked article states:

"The US argues that Google used its financial power to acquire potential rivals and corner the ad tech market, leaving advertisers and publishers with no choice but to use its technology."