The Logan Act (18 U.S. Code § 953) makes it illegal for U.S. citizens to engage in unauthorized foreign diplomacy, including to "influence the measures or conduct of any foreign government" or to "defeat the measures of the United States."
So Archie admits Trump is committing a crime. He's not allowed to act as a government official when he is not, thats literally a crime. He just compared trumps actions to legitimate diplomacy with Iran.