There are several orders of magnitude less people in C-suite positions than in MBA programs. If most needles are found in haystacks, it doesn't mean that haystacks consist mostly of needles.
"Over-representation" is also pretty vague and the leap to anything like "common" is completely unjustified. A paper co-authored by Hare (referenced below) cites a prevalence of 1.2% for "potential psychopathy" (couldn't find a number for strong psychopathy). So if 2% of C-level execs were exhibited potential psychopathy, then psychopathy would be way over-represented, but still quite uncommon.
