People have varying sensitivies to flicker, but the refresh rate of even basic cheap CRT monitors was something like 75 or 85 Hz, which most people found essentially flickerless. Higher-end monitors would go up to 100 or 120 Hz, one of the several ways that for some use cases they were superior to LCD displays for quite a long time. Televisions, at 50 or 60 Hz, were pretty bad of course.
The default flicker rate on this shader is 0.01 which is about 85Hz. Indeed it's almost imperceptable but adds a lot to the feel.
It was fine back then, but now I can't tolerate even a minute of CRT TV or low frequency monitor.