I built a spread sheet a number of years ago to ball park sizing stuff and to help gain insight in how the engine/fuel/turbo work together as a system. It is a steady state model with an engine characterized by VE, an intercooler characterized by a standard heat exchanger model with 3 equations and 3 unknowns, and a turbo characterized by a standard centrifugal compressor equation. It has a few of nodes and will calculate to convergence to balance flow from the turbo, the intercooler, and the engine based on the inputs. If it was calibrated with a dyno, it would probably give good results within a reasonable window of changes.
It assumes linear relationships between temperature and density, which I don’t think is 100% correct. Makes other very basic assumptions for airflow through the intercooler, does not account for any transient in metal temp of the intercooler or changes in airflow based on vehicle speed, basic BSCF assumption, and so forth.
The Banks guy seems to be making click bate titles the info is not wrong, the boost gauge is just one piece of info, but once you have a car sorted out, assuming no malfunctions and you are operating in a reasonable space for your equipment along with knowing ambient conditions the boost gauge is pretty good indicator from that point, at least IMHO. Plus, an engine is an open system, not closed like a metal box.
The example given is for illustrative purposes only just to show effects of charge temperature based on the intercooler and based on compressor efficiency. The changes are so large that I would only look them to get an idea of what could be going on. In reality, these extremes would turn the engine into a grenade.