The most coherent explanation I've seen is that the only power the executive has to directly impose tariffs are from some "national security" law (intended for stuff like sanctions against war criminals), so even though he wants trade concessions, legally he has to dress it up as a national security issue.
That said, he doesn't seem to care about any other limits on the executive, and I don't even know how this strategy was meant to get the desired results (is he telling the other leaders his real requirements behind closed doors?)
If that's the case then OK but it contradicts the point being made that he explains everything he is doing in interviews and does all of his negotiations in public on the basis of what he claimed.
If he wants trade concessions, which I agree is plausible, delaying the tariffs on only USMCA-covered goods is a significant hint that there's real backchanneling going on. I was very surprised to learn there's so many non-USMCA-covered goods, and one way or another it's fair to presume Trump is intentionally drawing attention to that fact.
But the idea that Trump has deep strategic goals he's lying to the American people about is hardly a defense. And all the information that's leaked about negotiations thus far is that Mexico and Canada are just as confused as the general public about what Trump is asking for.
That said, he doesn't seem to care about any other limits on the executive, and I don't even know how this strategy was meant to get the desired results (is he telling the other leaders his real requirements behind closed doors?)