Friedrich Hayek is generally regarded as the apostle of a brand of economics which holds that the market will assure the optimal allocation of resources... [and] whose two main pillars are the efficient market hypothesis and the theory of rational expectations.
This is usually called the Chicago School
No. Hayek was of the "Austrian school". The "Chicago school" came later [1]. It's really the philosophy of the Austrians that's he's objecting to. The Chicagoans arrive at many of the same conclusions, but do so not through philosophical inquiry, but through extensive reliance on mathematics and metrics. Much of what we see in modern econ derives from the Chicagoans.
Anyway, I don't think that he adequately rebuts Hayek. As Mises shows in Human Action, the human mind is a black box, and we have no way of knowing its motivations (frequently even our own). Therefore, it really isn't possible to say definitively that a person is not behaving rationally, and really the best model we can make (in the real world) is to assume that they are.
But even that doesn't matter. Because Soros conflates the Austrian and Chicago school, and only mentions the Austrian philosophy, he completely fails to rebut the Chicago arguments. So he hasn't really done anything to discredit a laissez-faire approach to economics.
[1] See especially discussion of Milton Friedman, https://secure.wikimedia.org/wikipedia/en/wiki/Milton_Friedm...