Earlier quoted context omitted.
There's some information asymmetry. A $24 pizza with free delivery might look to a consumer like a better deal than a $15 pizza with $9 delivery because the consumer can't tell that it's the same pizza. The $24 pizza might be more expensive because they use better ingredients, and the delivery might be free because of a loss leader. Or at least it might look that way to the consumer.
Another, probably cleaner way around this information asymmetry is the reviews that already exist on these apps. If you order a $24 pizza and something the quality of Domino's arrives at your door, you're not going to enjoy that place and you're not going to rate it very highly. There are plenty of problems with review systems, but there are problems with price caps too.
Because we do see this happen, which suggests to me that reviews are not effective enough in practice to overcome the perception advantage of the information asymmetry.