The platforms installed every 10 feet were presumably fir saplings? approximately 12-16 ft long and say 4-6 inches in diameter. Were they cut on the island, and if so where are the tree stumps?
Thems bigger than saplings. Thems posts.
Pine rots fast. Softwoods rot before hardwoods. Cedar can hang in thier but feel free to study their cut stumps lifespans.
I've removed oak stumps from green to decades old in this yard.
Had family helping last spring on fresh maple stumps, sassafras and oak.
And I dug and cut down the road a couple years ago in a yard there.
When I state leaving a stump suits me I mean it.
Then I go work another one anyways.
Stumps snag plows. Trip livestock. Chock wheels.
How they are treated after cutting factors in speed of deterioration. Great whitepine stumps with much sap/resin and exposed to air last decades.
I bumped a decades old oak stump (big one) with a box blade on a tractor by accident and it is free. Pounded on others that gave up too.
But Oak Island glory hole timbers stumps? Long gone had they existed.
How they became gone can vary. Stump pulling machines powered by horse/mule /oxen worked. Lots of other ways to pull them.
Or burn them. Or salt them.
Small post sized pine type stumps though would fade fast. Seems odd but if you can dry roots (see salt/nitrate use) a stump fades faster. Smaller stumps=smaller root volume. Smaller integrity after cutting.