The machine he is using is a plumbed in version. Looking at the pressure gauge during his shot I would think that the position he has the lever in opens the plumbing using the in line water pressure, but before engaging the pump.
Just my thoughts on the built in pre infusion you refer to, it is just the reaction time for the pump to get the pressure to the coffee puck. Every manufacture would be different in the brew path from the pump to the group head.
Just my thoughts, without knowing the Isomac or the expobar minore 4.