Yudkowsky's underlying epistemology is representational objectivism – There are some objective facts in the world; the job of the brain is to represent them with maximal accuracy and the job of communication is to transmit them honestly.
This is one reason I don't believe much in representational objectivism. Knowledge when looked through the lens of metaphor theory seems much more like a set of pragmatic tools for imposing meaning on a complex world than objective facts. Rationalists are aware of Lakoff but don't seem to have drawn quite the same lessons from him that I have. Which is that all of our concepts have an essential fictionality to them; and are never, despite our best efforts at science, simple objective facts.
It has a very particular theory of what that means, comprising a theory of knowledge (representational objectivism) and of action (optimizing aka winning). Both of these theories seem extremely weak to me, in that they don't adequately describe the natural phenomena they are supposed to be about (human intelligence) and they don't serve as an adequate guide for building artificial versions of the same.