There will be a million such near missed over the coming centuries, eventually one has to hit though. Although the odds of this one getting us are astronomically small, as the authors said - the risks are extremely high. What if the world were to sit back and watch as this monster 'nearly' misses us, only to find out that our calculations were wrong and that it was indeed going to hit us. The end of life...
Sure, no need to lose sleep over it, but we should be doing a little something in preparation.