Amazon did not buy Grubhub in July 2022. Its agreement gave Prime members a year of Grubhub+ and Amazon the option to take a 2% stake, with room to earn more if customer targets were met.
The trade was distribution for Grubhub and another benefit for Prime. I would not call it a turnaround until Prime members placed repeat orders after the free year. If the promotion only moved users from one subsidized membership to another, Grubhub bought a temporary spike rather than a stronger business. DoorDash and Uber Eats were already taking share.