I think the person's point was that the CBP agent said when a passenger has GE, he or she does not use up one of the airline's Pre-Check "slots." In other words, getting Pre-Check via GE membership leaves open more slots for non-GE elites, per the CBP officer.
I have no idea if any of this is actually true. Just clarifying what the OP seems to be saying here.
Ah, that would make more sense.