What the heck are we even talking about here? Smarts, or intelligence, are on a spectrum, and there are all types of ways to be "intelligent". MD students (and later, MDs) are typically highly motivated, task-oriented, and good at memorizing and synthesizing large amounts of information.

Plenty of people who wouldn't excel in Med School run businesses, build things, manage investments, invent things, etc. They're just as smart, in my book...