It's not that it is affecting maximum injection pressure it's that it is able to scavenge the volume of fuel to inject sooner with the plungers opening further. This is my understanding from what Mel explained to me, and I trust a experienced pump builder like Mel over a dude on YouTube with a diagram.
And that's true... AT WOT!
The plungers aren't going to open further /without the fuel getting to them/. This will only happen at full throttle; otherwise, how do you think you can vary RPM and torque?
Also... you trust someone's explanation over a cutaway of the device in question? It's not that he has a diagram(watch the video!), he's got a frigging cutaway pump! You can /see/ how it works!
Edit:
I may have misunderstood you a little.
The plungers do not have springs behind them, nor do they open by themselves. Fuel pressure pushes them open. The cam cam ring then pushes them shut. So, there is no "suction" going on anywhere in the pump - fuel that has "jumped the gap" into the rotor pushes the plungers open by the precise amount of fuel that is there. More fuel = more advanced timing, as the cam ring is fixed*
But, unless you get the fuel in to the pumping element in the rotor, there's no way for the plungers to fully open, because /nothing but fuel pressure opens them/.
(*The cam ring is attached to a separate plunger in the housing that moves based on transfer pressure - more RPM = more transfer pressure = more advance. So light load at high RPM is more retarded than heavy load at the same RPM.)