Intelligence in my view is the rate in which we learn. You can take this and relate it to any subject.

For example, a person who becomes adept at selling cars and persuading people is intelligent in this area.

Normally, people wouldn't include that in their definition of intelligence. They would say: A person who quickly grasps an abstract philosophical concept is intelligent.

They are (wrongly) applying one specific subject to the broad concept of intelligence.

Certainly that person is intelligent, too. But you've only stated what he is intelligent at.