There is no such federal agency as the "ATF", at least as it is referenced here, anymore than the Federal Bureau of Investigation is the "FI", or the Bureau of Land Management is the "LM".

The old BATF (Bureau of Alcohol, Tobacco, and Firearms) acquired such an egregious reputation that it started calling itself the "ATF". Then its name was changed to the Bureau of Alcohol, Tobacco, Firearms, and Explosives, so its acronym should properly be the BATF&E.

Not surprised that a BATF&E agent would flip-off someone, though. They've been flipping off the American public for many years.