As Tony mentions, the part you bolded makes the exception but says that they have to follow the second code that I included, which states that all vehicles used for a traffic stop must at least have 4 inch lettering. Since you have selective reading skills:

Quote Originally Posted by GA Code
Except as provided in subsection (b) of this Code section, any motor vehicle which is used on official business by any person authorized to make arrests for traffic violations in this state, or any municipality or county thereof, shall be distinctly marked on each side and the back with the name of the agency responsible therefor, in letters not less than four inches in height.