That’s true, but the GGP seemed to be arguing that the fact that treaties are defacto US law is sufficiently important to preventing the US from breaking an international treaty.
I agree with you and others that the driver for abiding by treaties is not US law but us reputation. But that’s the GP’s point: the law is not the main driver here.
if you gain a reputation of a country that does not honour agreements, no-one will be interested in doing agreements with you.