If I'd buried a treasure, it would be logical for me not to leave signs around encouraging people to dig for buried treasures. This would include a buried stone saying how much deeper the treasure would be.
Protecting my treasure buried in Oak Island with a stone in the wrong place is one approach. Another approach would be to bury a stone stating in very clear English that the treasure is actually buried in New York, and to go look for it there. An even better approach would not be to bury any stone at all.
An excavation of the sort that would have been required at Oak Island would have involved a good amount of planning done by intelligent people - the kind of planning and intelligence that would preclude some of the ridiculous absences of judgement that are required for most of the Oak Island theories to work logically. This is actually my biggest problem with the legend...it makes very little sense, and there were better options available.