A subsequently passed statute supercedes a treaty. It doesn't 'amend' a treaty because the US Congress doesn't possess authority to bind a foreign nation. If we have Treaty X with Canada and Conress passes law Y subsequent to treaty x and y conflicts with x, y prevails. Canada of course could obviously consider this a breach of the treaty.
A treaty has the same force and effect as a statute. I'm not going to debate this with you, its black letter law, look it up.
Therefore, if the US passed border adjustability, irrespective of whether WTO is a treaty or not, the tax law would prevail. My point is that other nations will look at this as violating WTO and respond accordingly, ie other nations will likely not permit the US to take protectionist measures with impunity.
The WTO is what it is. You can look up what we have ratified. Call it a treaty, call it an agreement, I don't care, either we abide by it or we don't, we're free not to abide by it in either case but consequences will occur if we don't.