Note that I didn't ask if she was less feminine - but less FEMALE.
I think you can see the prejudice at work here.
But let's take it a step further - among the listed tests they wanted to perform on Caster, who to them seems to be no more than a guinea-pig for their experiments, was a psychological evaluation. Why? So supposing she passes all these tests but happens to be more on the masculine side psychologically - what does that have to do with her suitability to compete with other females? If sexual orientation or gender identity becomes a deciding factor to compete in international sport, I think sporting is going to become a very complicated and difficult business indeed - with hulky he-men and super-model she-women on the racing track. And all the rest on the side-lines as spectators not fitting into either of the two categories.
The root of all this unhappiness and friction? IGNORANCE.
It is not simply Caster's humanity that is on trial - but ours.