No, it is not about knowledge, it's about belief (or any other representational state). An externalist believes that the content of a belief -- what a belief is about -- is determined by causal entanglements with the environment. I have beliefs about Eliezer Yudkowsky because my brain states are causally entangled with Eliezer Yudkowsky. These beliefs need not qualify as knowledge. Say a physical duplicate of me appeared on Mars by a random thermal fluctuation. Even though that duplicate has the same brain state as I do, his brain state is not causally entangled with Eliezer, so the externalist would say that he does not have beliefs about Eliezer. If you think this is obviously false, you are most likely an internalist.