I think you're misunderstanding the point they were making.
The given example for nice
> nice would be to let the person working for you continue to perform poorly without feedback, being "nice" to them, but privately considering them incompetent.
Is actually being hypocritical, not nice. The nice person would still tell the under performing person what's what, they'd just not be rude about it.
You can also create a scenario in which being kind becomes detrimental if taken to the extreme. However, the author of that book decided that being nice was bad, and being kind was good. This understandably continues to annoy people when this frankly dumb definition is brought up.