Don't follow your full argument, but I think get the point: the computer is expecting 50/50 hetero and homosexual people, after looking at the training data. However, the human judges likely assumed that the percentage of homosexual people in the study matched that in society, roughly 4%, and would bias their guesses guessing that fewer people were homosexual.

Assuming that's true, no wonder the computer did better. I'd bet it would do far worse dealing with a more representative sample.

Still, even though it's not a perfect study, it's still impressive it can do as well as it did.