Google ex-divisio would place ads with and display them from the divested newco just as Google's customers do today. Customers such as (checks notes) Facebook: https://mlexmarketinsight.com/news-hub/editors-picks/area-of-expertise/antitrust/google-acknowledges-it-foresaw-possibility-of-probe-of-jedi-blue-advertising-deal-with-facebook

The suit doesn't claim Google's other businesses are leveraging an illegal monopoly, but the part of Maps which does is their ad business.