I notice PWM all around me all the time. Lots of cheaper LED lights use it, and I see it whenever I move my eyes, also when not consciously looking for it - instead of a blur I see series of lamps as a brief afterimage.

As a child I got headaches from CRT monitors at 60hz and I could see them flicker. I haven't noticed the PWM on modern monitors, but maybe these Apple ones would trigger me?