I think we all know there is no such thing as an international governing body of truth. :D
So let's get to your real question: How is diplomacy possible? (under the tag political theory)
My dear fellow, read the book!!!
It is not only possible, it IS diplomacy.
In Art of the Deal, President Trump advocates the strategy of making BIG BOMBASTIC openers to stake out position and create room for maneuvering during negotiation.
This is a strategy he uses time and time again, to telling and stunning effects.
During his campaign he said he would build the Wall and Mexico would Pay for it. Does anyone actually believe Mexico was ever going to pay for the wall? Of course not. But he made a big claim, he will build the wall AND Mexico will pay for it. Now he is building the wall, and no one cares if Mexico is paying for it.
He said he would flatten North Korea Multiple times. And that brought Kim to the negotiating table.
With the fentanyl position, it is actually genius. If China doesn't do a deal, he can say "Those Chinese, those chinese, I am telling you, you have to believe me when I say, they want to kill us! They want to kill us with Fentanyl. And this is why we can't do a deal with them. (And people would applaud) And if there is a deal, he can say, "my good friend President xi, who i deeply respect, who loves his country as much as I love mine, has agreed to a trade deal. And it is a very good deal, believe me, a fantastic deal." (he then has the option to talk about fentanyl or not) Genius!
This is a different kind of next level diplomacy, and no one should take any of his claims overly seriously. If you do, you would go crazy and lose to the stable genius.