I don't know, to me he's just stating that the brain is the seat of sensation and reasoning.

Aristotle thought it was the heart. Both had arguments for their respective positions. Aristotle studied animals a lot and over-interpreted the evidence he had accumulated: to the naked eye the brain appears bloodless and unconnected to the organs; it is also insensitive, and can sustain some non-fatal damage; the heart, by contrast, reacts to emotions, is obviously connected to the entire body (through the circulatory system), and any damage to it leads to immediate death.

Also, in embryos the brain is typically formed much later than the heart. This is important if, like Aristotle, you spent too much time thinking about "the soul" (that mysterious folk concept which was at the same time the source of life and of sensation) and thus believed that the source of "life" was also necessarily the source of sensation, since both were functions of "the soul".

Hippocrates studied people more than animals, did not theorize too much about "the soul", and got it right. But it would be a bit harsh to cast that as a triumph of rationality against superstition.