This seems to be already the case in India with the Dabbawalla system [1], so it's not an outrageous idea - it's simply replacing home cooks with tiny commercial kitchens. They can buy produce in bulk at much cheaper rates than you'll find at your grocery store.
It's not the case, because it's not possible for food to be cheaper after prep and delivery. It's cheap, but not cheaper than making your own food from scratch. And there's no oversight or health inspections for this mass prepared food either.
But you know this, it's why you said "seems to be" instead of "is," very nicely done.
You're completely ignoring the food cost I just mentioned. For example, the cost of a Big Mac is around 1 dollar. Try making a burger with ingredients from the supermarket for $1, it is impossible. Things easily cost 5-10x as much in retail vs their bulk prices, it is much cheaper to cook for 50 customers at once than for a single meal.
This system is removing all of the other costs of operating a restaurant, so unless you also buy ingredients in 50kg bags for your home, they can definitely make it cheaper.
[1] https://en.wikipedia.org/wiki/Dabbawala