You mean "the outlandish argument that the Supreme Court is making", of course - this is not "my argument", this is the law.
...contrary to Article VI of our Constitution, would in effect hold that the U.S. was unilaterally abandoning the Geneva Conventions. The geopolitical ramifications of which would be incalculable.
Oh, well. You probably also won't believe me if I tell you that, constitutionally speaking, the US can withdraw from any treaty at any time by a simple act of Congress. But it's true, nonetheless.
Congress is an entirely different Branch of our government than is the Supreme Court.
What you were erroneously trying to say was that the Supreme Court could nullify a legally binding, Senate-ratified treaty.
That's a different thing altogether from that of Congress killing a treaty.