I don't think 3.5 was particularly lethal unless you had a killer DM. It wasn't a cake walk, but it definitely wasn't the most lethal game I played.
3.5 was only lethal when either the DM min/maxed the monsters like they were a PF sperg, and/or because the DR on some monsters was fucked.
we had a near TPK because the DM put against against some demons that did AOE to everything in a 45-foot radius when they took damage, and that was further than most of my bard spells would reach and our paladin's abilities. We could have probably whittled them down with ranged but neither the players nor our characters were feeling like playing I'm not touching you.
People who want to believe kickass women armies were just as capable as men will hang onto any myth they find.
Training and weapons make a big difference. As does good nutrition. An all female army marching with heavy gear or fighting for long periods... not so much. But a minor noblewoman who has actually been instructed in weapon use and provided with quality gear? I don't see why it's so implausible she couldn't fight off the odd roving peasant soldier. Even if she's fighting someone also professionally trained and equipped, there's a big difference between a woman fighting me and we're both unarmed and that same woman fighting me when we're both holding swishy 1m swords of death.
The two main issue with female armies is "Odds" and "logsitics"
There is nothing amiss with a city guard or guardscorp of females. Especially when its a royal guard unit where you have a battalion or less of female soldiers recruited from a whole country, you can find enough the mutant super stronk amazon women to pad out a unit and keep it supplied with new recruits. But trying to find enough women to keep an active campaigning unit supplied would be a hard undertaking. You need the top 0.1% of women to match the average man on raw physical strength.
<insert table comparing women's athletic world records to men's rankings here>
the other issues is logistics. Women just usually were kept away from the fighting because they needed to make babies which was a pretty lethal pastime, to the point Spartans would only give graves to men who died in combat and women who died in child birth, because both died in service of Sparta.
But women also have periods, which in addition to loss of blood requiring extra nutrition... if you are campaigning and the enemy knows your army is going to be significantly less combat effective 1 week out of 4, your enemy can leverage that. Now this is much less important when its a homeguard situation.
I guess what I'm saying is I give zero fucks if a player wants play a female fighter (ok, I give some concerns if its a guy wanting to play a female and I'm getting ANY bad vibes). You can be that one-in-ten-million amazon mutant. I do care when source books start injecting female footsoldiers into armies for diversity reasons.