Not really. It depends more on how you interpret the phrase "the dumb ones." It's clear that if the IQ distribution of prisoners skews lower than the general

Not really. It depends more on how you interpret the phrase "the dumb ones." It's clear that if the IQ distribution of prisoners skews lower than the general population, then we only catch "the dumb ones" with respect to the general population. What isn't necessarily clear is that we only catch "the dumb ones" with respect to the IQ distribution of all criminals. (I don't think we even know or have any good way of determining what the IQ distribution of all criminals even is, do we?) It's just another instance where plain language can fail to be precise.