I believe he is wrong on the international law aspect of it. That is the point of the letter. They will be making an agreement with Obama, not the United States. It is not a matter of domestic law being in conflict with international law. Obama's authority to enter a binding agreement is defined by the Constitution. The Constitution requires treaties to be ratified by the Senate.
Daily news- "We strenuously condemn their (GOP) betrayal of the U.S. Constitutional system." Where the F was their dissatisfaction the last six years while Obama was wiping his ass with the constitution????
Either that or the treaty is still in draft and it would be moronic to give it to the full Senate for ratification now.