That's an incorrect reading of that statement. Imagine the author had written "those who commit sex crimes are more likely to offend". This sentence is confus

That's an incorrect reading of that statement. Imagine the author had written "those who commit sex crimes are more likely to offend". This sentence is confusing or trivial, because those who commit sex crimes are offenders by definition. So the author could write "those who commit sex crimes are more likely to offend again". But then according to your logic, this couldn't be applied to the general population since they hadn't offended yet, so could never "offend again". If we are considering a baseball team down with a runner on third with two outs in the bottom of ninth, and of the next two batters (Jackson and Smith), only one has gotten a hit today (Jackson), would you consider this sentence trivially true?: "Jackson is more likely to get another hit than Smith?" Because Smith can't "get another hit"? No. And as I've already addressed elsewhere, the comparison to other convicts doesn't make sense in the first place for the author's argument.