There's always the possibility he made it all up. It happened. Actually...STORY TIME!
One (Belgian) sergeant-major named De Bruyn took the huge gamble by saying had been no less then a chef D'escadron in a French cavarly regiment, and was therefor given the same rank in the NL army, which meant he became a major (By law, Officers had to be given their old rank or a promotion). For some reason, they didn't find out and nobody ratted him out (or just nobody of his old regiment went to join the Netherlands army, which is more likely).
How did it turn out? Well, he was promoted to full Colonel and given command of the 2nd Carabiniers and served at Waterloo. He and his regiment fought splendidly and the 2nd is still seen as the best cavalry regiment of the NL army, and possibly the best heavy cavalry regiment of the entire campaign, considering the only other heavy cavalry formations were the British Union and Household brigade - and we all know how much they fucked up.
Not sure what happened with De Bruyn after the campaign, but as far as I know he had a good military career. They didn't find out he lied until far into the 20th century.
Edit: And talking about Colonels, I just became one. How appropriate.
