intake manifold pressure

In your tutorial you say the intake has only atmospheric pressure. That would only apply to a non rotating engine. A running engine has negative pressure (vacum) if the valve train is operating correctly. Of course the purpose of turbo or superchargers is to create positive pressure which they will do provided the engine is running and working properly.

maybe he meant the intake side before the throttle body is where the atmospheric pressure is present.

Context is important - I would surmise, without knowing the specific instance, that they were discussing the pressure available to fill the cylinders?

As you say, there is a depression, relative to atmospheric pressure*, within the engine that is drawing in the air from outside, deepening towards the cylinders. This causes a pressure gradient which allows the ambient air outside the engine to be pushed in.

Ignoring pressure wave tuning, at best that's a difference of 14.7 PSI available. With forced induction that can be greatly increased with the benefits of faster, and denser, cylinder filling.

*normally referred to as "gauge", while absolute is relative to a vacuum, and is easier to work with.