The aero-dynamics of a flight in reduced gravity are different than a flight operating in normal conditions. When an aircraft is experiencing zero gravity, it does not stall because the wings are not generating any lift, because the aircraft is in a state of weightlessness.
In the paper for the requirements for zero gravity flights, it is mentioned (PDF):
... zero G flights stalling conditions are different as compared to
conventional flight maneuvers, because the plane is weightless and is,
therefore, in no need of aerodynamical support. On the other hand, a
minimum speed is required to execute the delicate control through the
parabola. For this reason the stalling speed is taken as the minimum
aircraft velocity required for effective control.