> It relies on the idea that these authors were women who took pseudonyms because of the society they were in rather than fully by choice.

There is zero dichotomy between the two. Of course peoples choices are dependent on society they are in - both unconsciously and consciously as they weight consequences of their decisions. "Fully by choice" does not mean "you have to act like autistic person with no knowledge of society you operate in".

> Wouldn't using a masculine name and wearing masculine clothing be grounds to consider her in fact a him, a transgender man?

Not necessary. You cant translate our society into them that straightforwardly. Quoting wikipedia: "Sand was one of many notable 19th-century women who chose to wear male attire in public." The reasons varied from practical to basically expression of radical feminism. It also allowed her to go to places she could not go to otherwise.

She might or might not be trans, but you cant judge it from this that easily.