Argo,
At no point in the article does Col. Frederick E. Roseman state that he is a Marine. In fact, the only reference to branch of service is when he states that he asked to process the paperwork for the gun himself since "it would allow me to visit some of the Army units I had not previously seen (this wise move I was to regret a thousand times later on)".
I can't understand why a Marine officer, nearing the end of his tour, would want the chance to visit Army units of any type, but, there is likely much I don't understand about Marines. From the context of the aritcle, I would assume he was Army, and I do know no Marine I ever met would allow anyone to think for an instant he was Army.

Best,
Ted