Do not agree. My 08 Sprinter had a 3 1/2" x 4 1/2" hole cut in the floor and a Maxxair fan installed on the roof. You obviously can not cool the van below ambient outside temperature with holes in floor and roof. You can keep the van close to the outside ambient temperature with the holes.I may not be understanding your question in the right context, but if I am, the only way to cool a van in the sun is to have an air conditioner.
I could open the slider door and start to get in the van and I immediately knew I had left the holes closed. Very obvious. The natural draft of heat rising works.
I did it in my Sprinter because I spent some time studying passive solar house design before I built our house and barn. Both have low on wall openable windows and high at the roof peak openable windows. We do not have a normal sheet rock ceiling at 8'. Both buildings have high open ceilings that match the roof pitch. In the house the high windows are electrically powered. You can sit at the low on wall window and it feels like there is a electric fan pushing air into the house. Air goes in the window and out the open windows at the roof peak. In the barn I use a ladder to open the windows in the Spring and they stay open until late fall.
So with the house and barn as an example I could not see why it would not work in a van. It works. Not as well as in the house because you do not have as much elevation change that is available in a house to create the draft. But it is immediately apparent if you leave the van in the sun and forget to open the holes. Actually works better in winter because you have a source of cooler air.