So the 3 original men found the stone at 18 feet and it says "10 feet below...", instead of later diggers finding it 90 feet deep and it saying "40 feet below...". The map was 17th century, the map was 18th century. It was discovered, it was inherited. There is absolutely no consistency in the story which makes every bit of it unreliable. To my knowledge, the map was never a part of the earliest versions of the story so most likely it is a later embellishement.
Also consider human nature. The young son of a man who sailed with Kidd is handed a treasure map, and then never looks for the treasure despite the fact that he falls on "hard luck" and becomes a "public charge" in his old age. It makes as much sense as a dirt poor Appalachian being a sentinel for a KGC treasure cache. Sorry
@freeman, I give the map a very strong "tain't likely" rating.