I'm sure the work ethic gained from 10 years in the USMC helped
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.
That said, I'm inclined to agree with your last statement...