I understand that the author understands that "that demand" is tourists. What I meant is that you can't measure the amount of demand (tourists) by looking at the amount of supply (restaurants), which the author does.
Seeing that a certain level of supply remains stable over time tells you a great deal about the amount of demand present.
Of course it does. Much like looking at the skyline allows you to make some basic assumptions about aggregate demand for office space in Manhattan versus Kearney, Nebraska.