re: Your argument isn't logically sound
When you have George Marshall and Joint Chiefs of Staff adamently against something, it should tell you something. When have "to talk the entire U.S. delegation out of resigning." Something is up.
It actually makes sense to me that a US President could decide to do something which is against US foreign policy. But if you equate US President as US foreign policy, then this wouldn't make sense.
I think of it as there being a US foreign policy before the President shows up Jan 20th. I see the president as adding to US foreign policy or changing foreign policy or having an impact on foreign policy. And see a continuation of this foreign policy after the President leaves office.