Ok ok, so the experiment actually just tested whether an individual frame of a high frame rate display could be read while while falling. Since it could not, we do not have objective evidence for the subjective "time slowing down" experience. We just know that there was no difference from the control, i.e. subjects were unable to read display at high frame rate under normal conditions.
That's all you know. Sure, maybe stressful conditions make it harder to read, but then again the watch is stationary with respect to the eye while falling.