One of the most obnoxious things any human being can do is float some outrageous or insulting idea and then walk it back by calling it a “joke.” Trump does it all the time. He did it again with his suggestion that anyone who didn’t applaud him at the SOTU is a traitor.
I’ve seen the film clip. He certainly didn’t appear to be joking, and there is no reason to believe that his followers understood it that way.
This is a perfect example of how you can expect him to behave if there is a major war or terrorist attack, and next time, he won’t even pretend to be “joking.”