That's false. 20 is enough that you can see it as animation, but it's easy enough to notice it's choppiness (or at 24, or even 30, 40ish seems to be the average limit for the center of the eye for most people) if there's something fast moving on screen (or to notice it's excessively blurred).
Flickering is a different problem than smooth motion, noticing flickering in 40~60hz is not unusual, particularly at the edge of the vision. The edge of the vision is really good at fast moving things, unlike the center.
Noticing something appeared and then disappeared in a minuscule amount of time? Now that's a completely different problem, and well, we can, indeed, do that at what would be absurd fps if converted. We're talking 1/250ths of a second here.
Lastly, responsiveness adds another massive factor, which we can notice well above 24fps, and with games it's often the case that everything is tied to the fps, not just how long it takes the character to visually act, but also in sound, which we process much faster than sight.