In fact, 50% of people have a below average IQ, even when not under stress.
median != mean
Actually, median == mean for IQ, and for any other thing that follows normal (Gaussian) distribution.
Depending on the context, any representative measure of central tendency, such as the mid-range, median, mode or geometric mean, can be called the average.
A weird point to make, because in the context of a measure specifically constructed to have a normal distribution (IQ), the median definitely is equal to the mean. You are right in the general sense, and wrong in this instance.
Sorry, let me rephrase:
"The vast majority of people are far less intelligent / sophisticated than their own normal when stressed."
I got it - was making a sort-of joke - scams don't need to be that sophisticated or work on everyone, because exactly half the population has a below average IQ.