They could possibly be correct as they should know their own product better than I do.
I have to believe that the pump would have some safety valve or switch to protect it against over pressure.
If the pump isn't working then you may have to explorer a little deeper to find the solution. For example, what switches the pump off when the water in the tank is full? PF use a float switch which I presume uses a mirco switch and a solenoid valve.
Is the power supply delivering 24 volts to the pump? If you put a multimeter across the wires to the pump which has been disconnected from the power supply, is there circuit?
ps
I found this in the description of the 8800 series booster pumps.
" A 3 chamber diaphragm pump, with 3/8" John Guest ports and a 24VDC motor. It has a bypass set at 110PSI, and a pump body made of nylon.
It delivers around 1.9 LPM open flow, and around 1.1 LPM at about 80PSI.