absence of evidence is not evidence of absence
But the absence of evidence is the absence of proof.
I totally agree that there was a bias against showing women in command during TOS's run. Trekverse-wise ...
In Enterprise, there was a Captain Hernadez. But there was a century of separation between that and Turnabout. There was a Female Captain in The Voyager Home (forget her name), but there was a good dozen years separating that and Turnabout. The 1960's writer (writers) of Turnabout could have imagined a future with no female Human Starship Captains because there were no female naval ship's Captains in that present, certainly none in the US Navy. We believe there will be Female Captains because we have them in our society today.
Even today, a women in command of a US Warship is rare.
But what we have are two groups projecting their current societies into the future. In the time period of Turnabout, Human society could have been in a time period where there were in fact no females in Starship commands. It might have been that way for decades. Women could perhaps have obtain the rank of Captain, but only through non-"fleet" commands.
Societies change, it would be silly to assume that what we have today will never alter from it's "current" form.