$20k has a very well defined (albeit complex) meaning: it means that the sum of the prices of goods consumed by the individual adds up to $20k. (Note that the figures I gave for other nations were adjusted for PPP, so price levels are adjusted.)
I can think of no mechanism besides envy which would cause income inequality to harm anyone. It's not as if the US poor lack some good or service which is available in other, more equal societies.
Note that the Spirit Level only provides a correlation between inequality and other problems. It does not show the direction of causality.