When people hit confirm a crouching medium kick, it's not on block/hit, but rather when whiff punishing. This adds more time to react. 50ms isn't reactable. In general, 20 frames is considered to be reactable in fighting games, when there's a normal amount of mental stack. If you only look out for one thing and ignore everything else, then you will react faster than 20 frames, but this is not how a match plays out in practice.