The time refers to the time from order to delivery, not the time the order will take. If they streamline the system well enough and have a large number of delivery drivers in each area they could have delivery take ~15 mins, 4 deliveries an hour is $40/driver. Not great but more than enough to cover costs and wages.
I doubt in most major cities with traffic you could manage 4 deliveries/hour/truck unless your volume was big enough to have very close-by drop offs. Factor in the time taken to unload from the truck and hand off to the customer and it's even worse.
One thing I don't understand is who is doing the delivering. I doubt Instacart is using their own trucks, and instead is piggy-backing off somebody else's infrastructure.
They're delivering things like groceries, I assume they're similar to exec (https://iamexec.com/) but specific to delivery. It's just "some guy" hired by Instacart to pick up groceries and then deliver them when they're told to, no trucks needed, bicycles or cars would be fine.
