As I understand it from the news reports, Donald Trump's comment was that North Korea would face "fire and fury" if it continued to threaten the United States, which is a bit vague. I would imagine in his mind he means a serious physical threat such as a missile landing off Guam, rather than localised missile launches or verbal threats, which are pretty standard from Kim these days. You would certainly be right that if North Korea did aim an attack directly at or near US territory, and if Trump didn't respond, then he could indeed be accused of empty rhetoric.