Quite often during the Civil War you can find throw backs to the Napoleonic Wars in terminology, references, etc. This is probably where this comes from, and considering how much literature was published even by then on Napoleon's era, people were very well aware of it.

Civil War armies did raise and field light infantry formations - sharpshooters battalions and regiments. The Union had the 1st and 2nd Sharpshooters Regiments, Birge's Western Sharpshooters (originally the 14th Missouri Infantry, later the 66th Illinois), etc. The Confederates also raised sharpshooter units. Both had the same missions as French legere regiments.

Greg Biggs