manifold prussure, is exactly that, pressure in the manifold, or what we call "boost" when its past positive pressure or absolute pressure
where the pressure comes from is irrelevant, ie 1 turbo, 2 turbos, 3 turbos, supercharger and turbocharger etc
and pressure does not equal volume
this why 15psi on a stock turbo vs 15psi on a GT35R make different levels of power