FWIW, as a trans woman, I assumed that was for demonstration purposes to avoid showing anything intimate or identifying. Though some of us do remove essentially all hair south of the eyes.
Thicker and longer arm hair is still a masculine trait. Testosterone thickens hair all over your body. Feminising HRT does revert body hair for some people (but not facial hair, it's a biological quirk), but usually not enough to where cis women are if you started out hairy
I feel like harrier arms on women isn't that uncommon, but that could also just be the people I hang around (they tend to be more punk/hippie than the average)