Interesting idea! The idea of castration increasing intelligence didn't come up at all during my research, so I don't really know.

I expect that historical eunuchs would have been more intelligent than their contemporaries to the extent that castration protected against infectious diseases which caused mental impairment (such as ergotism, i.e. fungal food poisoning). At the same time, it might be that eunuchs mainly acquired positions of power due to cultural reasons.