While the proportion of the population who are serving is low, the propertion who have served is obviously much higher, I'd guess at around 2-3%. So ex-servicemen are 3-4 times more likely to offend than the average population. However, a headline figure can be used to prove/disprove anything.
HOWEVER, hopefully it'll be a strong kick up the proverbial for additional support where it's needed and the additional funding required. Further, it should also bring about a proper study, with a comparison across the World and through time within the UK. Why are these 9% in prison? Is there a common cause or trend that can be identified? Are they more likely to come from a particular force? Are they more likely to end up in prison if they've seen active service? Is there a difference in offending rates between those whot take up PTS counselling and those who don't?
The report asks more questions than it answers, and I think the Gov should answer these questions with funding for a proper military-specific survey.