The law he cites is designed for pariah states, not trade partners. US Code Title 50, chapter 35, addresses “war and national defense” and it “may only be exercised to deal with an unusual and extraordinary threat with respect to which a national emergency has been declared.” The law also provides that the president must consult with Congress “in every possible instance…before exercising any of the authorities granted by this chapter and shall consult regularly with the Congress so long as such authorities are exercised.”
In other words, the case isn’t quite closed for at least a couple of reasons. First, Trump would have to consult with Congress, which he doesn’t appear to have done, and representatives may not see the need for declaring a national emergency. While the president has made it clear that he believes in expansive executive powers, it’s not yet evident that Trump can be the plaintiff, prosecutor, judge, and jury when deciding legal issues with severe economic and geopolitical implications.