What? Yes it does. That's how prices work. If UVM sold all of their land, prices would go down, but they aren't selling all of their land, so the prices are higher.
I never said "all land is being used productively." I said "land prices are the primary source of cost in high COL areas." One contributor to high land prices are all the things you mention: people withholding their unused land from the market.
No, there are not more variables than just the sale price. The price of land is the price of land, and it factors in all the variables you mentioned.