Firstly, Gini coefficient is based on income, not wealth. That isn't stated clearly. Secondly, there's absolutely no data on what proportion of income flows to which decile. The conclusion (income inequality isn't a problem) is simply based on an assumption that the rise in the Gini coefficient is based entirely on the wealth accumulated by founders, and that this is a good thing.
I think this post boils down to "people like me are creating a lot of value, please acknowledge it. Also there are no downsides to this accumulation of wealth". This would be fine if PG also didn't argue that policies like wealth taxes are harmful (http://www.paulgraham.com/wtax.html). It just seems like a desperate play to keep his wealth intact.