Again it's a joke, so let's not take it too seriously but I think the sense in which it is half is not cardinality (that would be silly, as you point out), but

Again it's a joke, so let's not take it too seriously but I think the sense in which it is half is not cardinality (that would be silly, as you point out), but density. Look at the probability that a random statement of length at most n is true. Since for every statement it's negation has practically the same length and the opposite truth value, the probability should tend to 1/2 as n goes to infinity.