when trump says he's a nationalist he's going for the connotation, so arguing about exact meanings sure is silly (or you wouldn't know a dog whistle if it busted your eardrums - dog whistles that sound like train whistles someone quipped - no kidding, he's not subtle). And what are the connotations, perhaps nazism, but I suspect he was more going for white nationalist (but aren't those the same thing? not entirely - white racism in this country preceeded the nazi's by a long time indeed).
Look if Trump wanted to say he was protectionist, he could have used that word. He could have said isolationist (not accurate as the foreign wars never end, but whatever). He could have just said he would prioritize U.S. interests. But he chose the words he did to appeal to those he did.