It is correct that boost is directly correlated to load. This is because load determines the airflow and fueling needs of the engine.
If you accelerate lightly and keep the engine in a low gear, you’ll notice that it won’t build boost regardless of RPM. This is because the throttle plate is barely open, so there is not a lot of airflow through the engine. Turbos spool on the principle of exhaust flow and the expansion of hot exhaust gas, so the turbines will be spinning but the compressor is not proving any airflow/pressure above what the engine requires, which is what it’s drawing in on its own.
When you put a load on the engine, such as hard acceleration or pulling a hill, the throttle plate angle is much greater (open). More air flows through the engine, and therefore more fuel is injected to keep the air/fuel ratio in the desired range. This means more exhaust flow, which spools the turbochargers up to speed, which can be as high as 200,000 RPM.,With the additional exhaust flow, the turbochargers pressurize the air in the intake system, since the turbos are able to provide more airflow than the engine can draw in on it’s own. The pressure between the turbo and intake valves is boost; as the intake valves open, air is forced into each cylinder as opposed to being drawn in my vacuum.