Why does it not surprise me that the Attorney General for the District of Columbia is ignorant of the laws governing the District of Columbia? The law says that "no person shall possess..." It makes no provision for intent to harm, or for anyone else's unfounded beliefs about that intent.
He got the purpose of the possession wrong, too. It had nothing to do with speech or debate. The sole purpose of publicly displaying illegal weaponry was to undermine and attack the Constitution of the United States, as represented in the Second Amendment of that Constitution.