The granite weighs about 13 pounds per sq. ft. The load is on the sides and back of the cabinet. If you were to lift the countertop off the cabinet you would see that the post of the lazy susan, while connected to the top of the cabinet, doesn't even make contact with the underside of the countertop.
In other words, if you were to drill a hole in the top of the cabinet you could stick your finger in it a feel about 1/2" space between the base cabinet top and the underside of the counter top. It's how most cabinets are made. Units with drawers are open on top but corner units like that aren't.
Just given the design of corner units, there is more than likely, no countertop weight on that post.
Look at page e28 here and I think you'll see what I mean.