The commenter just described the probability of tossing a coin 21 times and all cointosses being heads, so (1/2)^21, or 1 in 2^21.

It's a specific case of the cumulative binomial distribution which is what you might look up if you want to know about the case where the probability of people liking X in general isn't 50% but something else, or the number of samples isn't 21, or the number of sampled people liking X isn't 0.