Vance, whose entire world revolves around blind obedience to Trump, is surprised when foreign diplomats don't have blind obedience to Trump. Bet his only tactic in the room was "but Trump says to have to accept this..." Over and over.
The EU is also weird because 'international' flights inside the Schengen zone are considered 'domestic', so if you land in Paris and connect to Berlin you go through customs in Paris because that's where you entered the EU.
Places like Paris CDG get messy because you sometimes have to change terminals which means 'landing' in France, travelling to another terminal and going back airside again.
Most countries, including the EU, operate airside international zones so if you're transiting (as long as you're not leaving the terminal or your wait isn't crazy long) you don't have to go through customs.
Not to mention though traffic. If Customs is pulled from international airports that means changing planes, even international to international is impossible. Because the states is one of the only places where you need to clear customs just to change planes