He also claims to be the only person to know the location, therefore for him to verify that, he would have had to visit it himself. He's now 86 and probably not able to hike into the mountains to check. So perhaps it's not far from a road?
Or the treasure is of the form "take this letter to the bank in order to get your reward".
Or maybe he is confident it is not found because it can't be found because it is a hoax.
This is explicitly declared in the fine article. It's one of the clues referred to in the title.