That would explain the centuries old folk's tale that soldiers and sports man are generally stupid.

(high cortisol).

Except water sports, running and golf etc

Exercise helps moderate cortisol.

Competition can increase cortisol but it's usually a relative thing:

When you are world #200 playing against world #50 might be stressful. When you are world #2 playing world #50 is probably another day in the office. Etc

I imagine once you win your first Grand Slam in tennis the nerves improve a lot - you kind of saw that in Andy Murray who needed a couple of attempts to break through the psychological barrier of winning Wimbledon, including winning the Olympics.

I would posit that the old folk tale is more related to the fact that centuries ago, most people were hyper specialised and standardised widespread education was less of a thing.

You'll find plenty of intelligent people in all walks of life depending on how you choose to define intelligence.

I highly doubt the veracity of this claim.