Well, three observations I guess because I want to preface this by saying that unlike many of Don's posts I think the vast majority of this is just fine, and it's really just Econ 101 - there's nothing especially libertarian or otherwise unsavory about it.
But there are two other points I'd like to make.
First, when Don says that Bill Gates got his wealth by being creative and giving people what they want and not by making people poorer, I think he is giving too short shrift to issues of bargaining over surplus. In undifferentiated markets perhaps that's OK. When you're dealing with people of Gates's caliber I think that's less appropriate. We as economists talk about distribution in a lot of ways. A popular way of explaining distribution is marginalism, which is appealing because marginalism is what two rational people would do if they had reasonable objectives they were pursuing. That's fine and dandy for a whole lot of things. But even without getting into any Marxian stuff at all, we also talk in terms of bargaining over surpluses. This is the framework that a lot of labor market and household models use, and I assume elsewhere in economics. If agents bargain over the surplus they create together some agent could have more bargaining power than another, and you can very reasonably think in terms of taking surplus from someone - making them poorer by virtue of the fact that you are capturing surplus that they had claim to as well but did not have the bargaining power to obtain.
We shouldn't be afraid to admit this may happen, and we shouldn't be afraid to put away silly phrases like "well nobody put a gun to his head so..." etc. You don't become some anti-market leftist if you admit that bargaining over surplus happens.
Second, towards the end of his post he makes a lot of assumptions about the relationship between distribution and growth. His claims are very dicey. There is obviously a lot of interest in questions of distribution, but most of this work as it relates to growth in mainstream economics comes on the supply side, typically as it relates to human capital. I understand there's an institutional literature too. But Don is thinking on the demand side and as far as I'm aware the only people to treat distribution and the demand-side of growth really seriously are Post Keynesian economists. And in Post Keynesian models and empirics, this relationship is a lot more ambiguous than Don imagines it to be. It all depends on the behavior of the investment function, its responsiveness to profits, expectations, leverage, and whether the super-rich are really entrepreneurial or serving in more of a rentier capacity. The consumption function is usually in the background in these models, which shouldn't be especially surprising.
I would frame these points of departure from Don's main thrust as being a case of where he takes an Econ 101 sounding platitude and runs with it, but it doesn't quite work.