I wonder (no pun intended) how much of this is driven by Marc's residual dislike of Amazon?
Amazon currently has a partnership with Grubhub to provide Grubhub delivery as a Prime benefit.
Amazon competed strongly (some might say unfairly) many years ago with Marc's first big startup, Diapers.com, and effectively forced him to sell to Amazon.
He later started Jet.com and sold it to Walmart - in many ways as a second chance to compete with Amazon.