Probably so. It's not really a mistake; outside of the U.S. the word Yankee or Yank (or some linguistic variation) is always used to mean any American. I remember the Spanish version of the word (Yanquí) being used this way while I was still in Cuba. Only in the U.S. does it have the North vs. South distinction. A Brit using the term to mean 'any American' is completely justified in his own idiom.