You're leaving out alot of valid points if you're trying to argue the plausibility of a woman having higher than average upper body strength, as far as raw muscle mass goes. It's common knowledge that testosterone is a primary catalyst for muscle growth, and more traditional, 'masculine' muscle distribution. As is also common knowledge, you can get testosterone from eating red meat, beans, oysters, tuna, etc., and vitamin D is a large catalyst for its absorption into the body.
Thus, in a society next to the ocean (or bean farm), with plenty of work that needed to be done outside (in the sun), it's entirely plausible that the differences in testosterone between men and women are alot less extreme when compared to today's society (where you have the choice to, say, not eat oysters and stand out in the sun all day long).
Probable? No. Absolutely not. Plausible that something such as the Shieldmaidens could have existed? Extremely.
EDIT: Just to clarify, I am saying that it's possible that the conditions for their society, in that era, permitted women to consume much more testosterone than even a FTM transgender takes. It also coincides with the hypothesis that the human body actually relies on largely external sources for its testosterone intake, meaning in a very hunter-heavy society, both men and women would have similar physique.
