Yes, in the society and economy we are discussing this is a universally accepted principle.
Bob is a farmer. He has the same amount of land as his neighbor, but he works twice as hard, and grows twice the number of crops to sell in a year. He has twice the income. No one would reasonably claim that he's entitled to none of what he produced. Bob's greater productivity than his neighbor is a source of income inequality - he earns twice as much!
There is a mainstream view that society has the right to tax Bob's income, so he does not receive all of what he creates. That's the debate: how much is fair? Maybe it's fair for Bob to have twice the income of his neighbor, or maybe less than twice as much. Unless Bob gets the same as his neighbor, there's income inequality because of differences in productivity.
No farmer is going to singlehandedly earn a million times what his neighbor earns. However, because of technology, this is now possible in some industries. For example, Notch is a game designer, and he singlehandedly makes a game that sells 20 million copies. Notch now has a vast income that far exceeds the typical game designer. Is this bad? What is bad about 20 million people choosing to give Notch their money in exchange for his game? I don't see anything wrong or unjust about this.
PG's essay points out that income inequality is a complex phenomenon, with multiple causes, some that are good (small groups or individuals creating amazingly valuable things) and some that are bad ("kids with no chance of reaching their potential"). PG's essay from my perspective serves to make the point that perhaps not all income equality is bad, and that there deserves to be more thought on the topic of "how much is bad?" or "what causes of income inequality are bad?", instead of treating income inequality as a one-dimensional issue that is intrinsically bad.
From my perspective, PG's point is fair. Income inequality is not intrinsically bad: when people work hard, improve, and become more productive, they earn more, leading to income inequality. This incentive to improve and produce is good, in my opinion. Perhaps what we as a society should be tackling are bad problems like "kids with no chance of reaching their potential", rather than considering income inequality to be a problem ipso facto.