Having spent many years hanging around US servicemen of one ilk or another, and lots of US civ types, I'd say that this Sgt is full of the proverbial...
Back in 1971-75 as an air cadet I made many friends with members of the WA Wing of the Civil Air Patrol...the terms Canuck and Yank were thrown back and forth constantly, and usually with affection.
From 1975-78 I was with the Cdn Militia and wound up on exercises with a US Army Nat'l Guard Av Coy, a Green Beret Res unit, and a US Marine Res unit. Same song second verse.
I married a girl fm Boulder CO, and have spent a great deal of time hanging around with the cousins. Never once have I been ubraided by a US citizen for using the word "Yank", nor would I ever balk at the term "Canuck". I'm curious to hear if this guy that jumped all over you was US or Cdn. If he was US, well maybe he was just having a rough day. If he was a Cdn, tell him to get his PC head out of his PC ***, and get a life.
My two cents.