I'm wondering how he knows they were within 200 feet

He knows because people are making detailed accounts of their adventures. Many of these people even show up on local news talking about where they are hunting. The treasure is a secret. The exploits of some of the treasure hunters... not so much of a secret.

I saw that episode! Love Expedition Unknown :)

When I was watching the show, I was thinking that the letterboxing community could surely figure that puzzle out!