the theory i like (unsure if its mentioned in the wiki) is they built the land around it up as they built it so they "just" had to move the blocks uphill on logs etc to the stone (thats now even with the new ground level) they wanted it on. when that layer is done move more dirt in so its now even with the new level you just built.

then, just remove the dirt. poof. snimple if you have basically unlimited manpower.