I think they are implying that people with lower intelligence are more likely to be in jail. There's at least three reasons why I would consider this very plausible:

1. Intelligence is associated with higher impulse control.

2. Intelligent criminals are probably less likely to be caught. They might also have a better chance at working to lower their sentences after being caught.

3. It is harder to be successful with a lower IQ (although I would posit it is far from the most important factor), and economic hardship is strongly correlated with crime.