There are also bad people who are uncomfortable with power struggles. But in the context of the article, I suspect that the most important definition of "nice"

There are also bad people who are uncomfortable with power struggles. But in the context of the article, I suspect that the most important definition of "nice" is probably "pays me for executive coaching". The term isn't defined because "I am nice" is an axiom, "I can do what I need to get what I want" is the desired conclusion, and the article provides some motivated reasoning for getting from A to B.