If there were copper foil inserted between those two prepregs, the board would be a 6-layer one and priced accordingly. Not putting the foil there would spare them a number of technological steps, but the board would be equivalent to a 6-layer PCB with 2 internal layers etched off completely, leaving 4 properly spaced layers for HV routing. But that's not how they make 6-deckers, anyway.
Best regards, Piotr