Part of the argument is the Uber is building product infrastructure of their own - Amazon built AWS as part of their everyday working, Uber could roll out delivery, autonomous tech or even their financial services as separate strands once they've reached pervasiveness.
Indeed, Uber already has their UberEATS program where they offer home delivered food from restaurants that don't yet have their own delivery service. So they're already experimenting with providing delivery & mobile payment processing services.
Yep - much bigger fan of Deliveroo, but whilst UberEATS might fail, could almost see them being more of a delivery API and last mile brand than the full restaurant service stack.