Let's not confuse correlation with causation; I once looked around for studies showing experimental interventions which actually increased work ethic (specifically, I was looking for studies on the Big Five construct 'Conscientiousness') and didn't find any.
So, I'm sure the work ethic that let him join and last 10 years in the USMC helped.
Lets not confuse an absence of evidence as an evidence of absence - just because there's no study yet that shows that 10 years in the USMC helps with work ethic, doesn't mean that 10 year in the USMC does NOT help with work ethic (unless you mean to say in your survey of the lit. you found something claiming the opposite of the op's claim).
That said, I'm inclined to agree with your last statement...
Well, in general terms, if the USMC (or the Army, or the Navy, or...) really did build up work ethic - a personality trait that correlates quite a bit with income - why do veterans seem to do so poorly, even veterans who never experience the confound of war? Because personality traits are, like IQ, very tough to alter. (This is why that psilocybin-increases-Openness study got so much play.)
You do poorly at kicking ass & making bank in civilian life. This is a tricky subject, of course (any statistics from 2001 and on are basically worthless because all the suicides and whatnot from Iraq & Afghanistan trash the data), and there are all sorts of correlation/causation problems (maybe the people who join would do poorly in the first place - these days, most of the graduating class of Harvard heads to Wall Street and not officer training).
But if you have any citations to the contrary - that veterans do better than one would predict from their pre-military life - I'd be glad to hear them. Conscientiousness is as important as IQ, and any intervention which genuinely improves either is really valuable and ought to be well known.
Don't confuse work ethic with motivation. I know a number of successful veterans who are kicking ass and making bank in civilian life. I also know a ton who are just 'retired' and enjoying their low-key life with friends and family.
You can't switch from studies and causation/correlation to just stating without citation that they suck at kicking ass.
So, I'm sure the work ethic that let him join and last 10 years in the USMC helped.