These are qualitatively different kinds of relationships. PG functions as an advisor for YC startups, someone who can influence the direction of your life greatly for a period of time. The difference between failure and success of an early startup can come down to a few critical days or weeks, and living in a version of reality that your ego has warped to protect itself is usually death for a startup. The most important thing in this case is to break that psychological bubble as fast as possible, and harsh words are one way to do it.
One's relationship with a spouse or a child is different. You will generally have many years to influence their thinking, and they won't usually suffer if you do it slowly.