What this song renders
Onna-musha (女武者) and the related onna-bugeisha (女武芸者) are terms used — sparingly — in Japanese sources to refer to women of the samurai class who trained in martial arts. The terms appear from the late Heian and Kamakura periods onward. They were never common: Japanese feudal society did not normalise women on the battlefield, and the named cases survive precisely because they were exceptional.
Tomoe is the most famous of these named cases, but she is not alone. Hangaku Gozen is documented in the Azuma Kagami defending Torisaka castle in 1201. Later periods produced figures like Nakano Takeko, who fought at the Battle of Aizu in 1868 against Imperial forces. The tradition is thin but unbroken.
What Tomoe does for the term is concentrate it. When later samurai literature, kabuki, and modern fiction reach for an image of the onna-musha, they reach for her. The word and her name are not synonymous, but they are inseparable. The song is the recognition of that fact.
The term onna-musha is real, used historically, and rare. Tomoe is its most-named exemplar. Hangaku Gozen and Nakano Takeko are independently-documented later cases. The song renders the inheritance the word carries.