The legal basis of the linked class-action is the Trademark Act of 1946, and the claim is that Grubhub has infringed on the claimants' trademarks by presenting

The legal basis of the linked class-action is the Trademark Act of 1946, and the claim is that Grubhub has infringed on the claimants' trademarks by presenting itself as acting in cooperation with the affected restaurants, even though it has actually been acting unilaterally. So the claim is less a middle man saying "We'll deliver you food from Lou's Diner", and more putting out ads for Lou's Diner with their own phone number in and delivering from Lou's Diner