Very valuable insight.
However, (b) in its own right, can serve as a fast growth business model - where the delivery or "clearing" of value between those who demand and those who supply is the value proposition itself - because everyone wants delivery (making it, by default, a big market).
Most banks work on this principle, in an abstract sense. A business like FedEx or UPS is a more physical example of this.
Online takeout-ordering services are examples of this - the customer wants the food and the "online ordering website" startup does not produce food - but what it produces is "clearing" ie matching demand to supply.
This - as an idea, in my experience, always scales and grows fast as well while falling into the category (b) that pg mentions.