It is basic, hickskateboarder....but it does illustrate that it would be hard to undervent the stock airbox lid even with mods.
Personally, I like the idea of having a vented lid as a first layer of protection against mud splashing onto my filter.
Think about the diameter of the carburetor...doesn't that really pose the restriction on how much air the engine can flow no matter what? Even if you had double the area of the smallest inside diameter of the carburetor to overcome any frictional losses or turbulence in the stream, you would still not have an airbox full of holes...
I will say that this formula is intended for naturally aspirated motors, not forced induction motors for sure...
If I lived and rode in a less muddy area, I would probably not run a lid either.
Either way, as others have mentioned, the motor still needs to be rejetted if the amount of air flowing through the carb has changed...