Yep, the Royal Green Jackets were formed well after the Napoleonic Wars. Specifically, in 1966. Which is certainly a whiles after.
But if a regiment wants to be historically inaccurate, they have 100% the right to do so. Many regiments do historically inaccurate stuff, and some regiments are totally historically nonexistent. Since this is a game, and not so much a simulation, who cares?
I'm pretty sure the reasoning behind the naming of this regiment was the Empire: Total War unit, which was veerry historically inaccurate for that game, which otherwise kept pretty solid historical reference.
