There are a lot of other plausible explanations & problems with the study, some, including yours, were raised in the article I read on the study a few days ago.

I can't find the exact one, but these should suffice:

http://www.theatlantic.com/health/archive/2012/09/study-castration-adds-...

http://www.npr.org/blogs/health/2012/09/25/161746488/korean-eunuchs-live...

It's actually a main problem of the field of history: when you find a nice, pretty complete archive somewheres, you can spend a lot of time looking for patterns in it, and find out some neat stuff, but generally it's folly to start getting any illusions that you are getting at "the whole story.."

Maybe not oddly, because good ones have good detective skills, but I have found that a lot of medical doctors are often big fans of the hobby of trying to diagnose long-dead historical figures and other historical health issues. Years ago when I had a teeny studio on the upper west side in Manhattan, on recycling day I used to sometimes pick up stacks of medical journals discarded by the many doctors' offices around. One of the main journals (like JAMA) had a regular column doing just that which ran in the front pages (maybe it still exists, I don't known) which impressed me like: "here's your fun stuff before you have to start cracking on your homework." It would be something like "what was Alexander the Great's real cause of death?"