Not a lawyer, and I don’t even play one on TV, but I am pretty sure the protocol of addressing an attorney general as “general” has been in usage in the courtroom long before anyone ever heard of that TV show.
What about Postmaster general, surgeon general
I never heard of that usage ever until the Seinfeld episode in the 90s, in which it was part of a joke. Then it kind of seeped into usage a little, and is becoming more common. If you can find a reference to an attorney general, postmaster general or inspector general ever being referred to as “General X” as opposed to “Mr. Attorney General”, I will be happy to look at it. It could be that the mistake was made sometime prior to Seinfeld, but it would still be a mistake. We don’t have Attorney Colonels or Attorney Lieutenants, and we don’t have Attorneys in charge of the Justice Department who ARE generals. They are THE Attorney General.
Think about it. What would you call Rod Rosentein? General Rod? He’s the Deputy Attorney General. That doesn’t make him a General any more than Barr is one.
They are not part of the military and the salutation “General” is inapposite.