I don't think he served
The chances are he did serve, but in a very different role. We had and still have plenty of Walts from my time in green, perhaps the most famous being
Baron Castleshort. His time with 21 was very short indeed, but look what he did with it.
Many of the Walts have been close to the role they claim and so have been able to pick up some jargon and some believable stories about their "when I was on xxxx".