A nice fancy flying in the face of physical realities and psychological tendencies.
If you 'let' strength decide, you don't end up with conveniently iconic femme fatales in charge, you end up with history. i.e. the occasional aberration or martyr/figurehead like figure... not a culture embracing the idea, at least not until society reaches a level of sophistication and distance from survival scale realities to where 'strength' stops being so relevant.
Came the English early,
Or came the English late.
They always found Black Agnes
A-Standing at the gate.
History does not say what you think it says. What it says is that cultures which have not reached a level of distance from survival scale realities cannot afford to let half their population stand idle.
We know very little about the ancient Celts. What we do know, however, indicates that female fighters were not unheard of. Cuchulain's tale includes two, Scathach and her rival Aifa, and no comment is made on them being unusual. Ceaser mentions them too.
In other ancient cultures there are similar figures. For example, The ancient Gothic heroine, Hervor, appear in tales originating not long after the events they chronicle.
In the middle ages, women often engaged in warfare. A lord's wife's duties included defending her home if it was attacked while her husband was away, or dead, and women could inherit fiefdoms if they had no brothers. Being female did not exclude them from military service. Black Agnes, Countess of Dunbar, is one of many noblewomen who are known to have defended their castles against attack, and some led troops in the field, occasionally as supreme commanders. The period of King Stephen's captivity during the civil war that dominated his reign saw both sides being commanded by women (unhelpfully, both named Matilda). In King John's reign, Nichola de la Haye was Sheriff of Lincoln, and defended that town against the kings enemies.
Women who actually engaged in combat themselves were rarer, but by no means unheard of. They include Jean de Montfort and Isabel of Conches, and eyewitnesses of the First Crusade, both Byzantine and Arab, report (with dismay) western women fighting:
Among the Franks there were indeed women who rode into battle with cuirasses and helmets, dressed in men's clothes; who rode out into the thick of the fray and acted like brave men although they were but tender women, maintaining that all this was an act of piety, thinking to gain heavenly rewards by it, and making it their way of life. Praise be to him who led them into such error and out of the paths of wisdom! On the day of battle more than one woman rode out with them like a knight and showed (masculine) endurance in spite of the weakness (of her sex); clothed only in a coat of mail they were not recognized as women until they had been stripped of their arms
Imad al Din.
Females were numbered among them, riding horseback in the manner of
men, not on coverlets sidesaddle but unashamedly astride, and bearing
lances and weapons as men do; dressed in masculine garb, they conveyed a
wholly martial appearance, more mannish than the Amazons. One stood out
from the rest as another Penthesilea and from the embroidered gold which
ran around the hem and fringes of her garment was called Goldfoot.16
Nicetas Choniates.
Howard was almost certainly aware of this sort of thing, although we cannot tell what he had actually read of course. But he uses the idea in one of his historical tales... The Sowers of Thunder.
Now... that said, the view that they actually bore arms in battle alongside men is also valid in that line. It is however a still farther step from that to deciding what their actual role may be. Anything from auxiliaries fighting beside their men kin to 'troops' of the line... to accepted as a leader of men I suppose. But that last is one, and the least likely IMO, among many possible views as to what that could have possibly meant.
I think you have the liklihoods backwards. To find a woman in a command position is more likely than to find her as a fighter.
