CS Troops called them Billy yank because that a Nickname for the US Soldiers when at War.? this makes no since to me. regardless of the war CS troops would remember TR, if from nothing else the scary stories their dads tell them before bed. if this is in the SGW/WWII then they would be confused as to why these yanks are referencing a man two decades dead.
Like German called the Brits "Tommy" or Americans called the German "Krauts"
It will be normal to be called "Billy Yank" to the SGW Union Troops,
but not for OTL Troops who will highly to be bewildered with the Nickname