Yes, I get that. "Reverse ground effect" was how the OP phrased it. What exactly do you think would happen in that case? Suppose the available air were "cut off" by the overpass (it wouldn't be, but let's suspend disbelief for a moment). What would be the result?
Loss of lift due to the interruption of the airflow causing the helicopter to crash into the ground at speed? Or suddenly lower pressure above the helicopter causing it to be "sucked up" into the ceiling.
I think the pilot is probably correct that the stunt could only be pulled off at a higher speed where you can outrace the low/high pressure zones the blades are creating.
