I don’t think serendipity is equivalent to luck. You can’t create luck, but you can encourage serendipity. If you’re a hermit who refuses to get out of the house and network, for instance, your chances of serendipity are lower than those of a social butterfly.
In addition, the point here is not that planning is pointless. It’s just that it often is a waste of time. For some people who obsess over their careers, it’s a BIG waste of time. If folks take anything away from my post, it should be to “be present” in their careers, focus on what is in front of them, and every couple years pick their heads up and look around.