No he isn't confused with a nasp, on a nasp the inlet wouldn't be filled with boost pressure.

The fuel pressure rises 1:1 with boost pressure to compensate for the pressure on the injector nozzle.It also drops the fuel pressure in vacuum for the same reason.

if you had 40 psi of fuel pressure and 40 psi of boost, how much fuel would you inject?

With a base pressure of 40PSI
Idling at 20inHg, the nozzle is exposed to 20 inHg vacuum causing fuel to be sucked into the intake manifold. This combined with the 40 PSI base pressure would equated to a theoretical spray pressure of 50 PSI which is above the desired fuel pressure of 40 PSI. Since the top of the valve in the FPR is also exposed to 20 inHg, the valve in the reg is pulled up which opens the outlet port up increase the fuel being bled from the fuel rail which reduces the pressure by 10 PSI. This then gives you your desired fuel pressure of 40 PSI.

At a boost pressure of 10 PSI, the nozzle is exposed to 10 PSI of pressure. This pressure is acting against the base fuel pressure which would mean that the spray pressure would be 30 PSI which is below the target fuel pressure. By also providing the top of valve in the reg with a reference boost signal of 10 PSI, the FPR now has a combined force to provide 50 PSI of fuel pressure which can overcome the force exerted on the nozzle to give you your 40 PSI target pressure.