The so called science is off too.
The female arousal is, in part, measured by the degree of lubrication in the vagina, but most potop MtF's do not self lubricate. Only a very few of us get so lucky, so that would considerably skew the results... I also think it might matter how long postop the subjects were, what surgical technique was used, and indeed possibly even at what age they originally transitioned.
Besides from my own experience my sexual arousal varies at different times depending on all sorts of things, I am also basically bi, and indeed after so long postop I no longer find implied masculinity to be a threat so that also would make a difference.
All in all I think the idea was a clever one, but in practice postop MtF's as a group would have too many hidden variables that the researchers simply would not be aware of to be statistically valid as a control group.