(1) The reason is NOT because of physical strength. I've always detested this ridiculous claim. Sure, the average woman is less strong than the average man, but there are plenty of women who are stronger than plenty of men. And that's even assuming that all areas of battle rely on brute strength, which is simply not the case.
(2) A much more logical reason why the situation developed: for much of our species' history, any given group of humans grew in power and security proportionally to the size of the group. A larger band could send more people to war - and then, unlike now, pure numbers were likely to make the crucial difference in a battle. So sending women into dangerous situations made less sense because women were far more necessary in maintaining and increasing population. As the blog's author points out, if half of a group's women are killed, then the next generation born will be half the size of the last one. (We're assuming that the women killed include half of the ones who would otherwise be reproducing. Of course, in many bands of early humans, this would basically be all of the women of reproductive age.) If, on the other hand, half the men are killed in battle, the next generation could go a long way toward repopulating the group.
So women were excluded from battle for reasons which, while once practical if a community wanted to survive, are now totally vestigial. Still, the population issue may be relevant in many fantasy worlds, so the author addresses some ways in which writers might design worlds that need not bow to these reasons and exclude women from combat.
I appreciate the author's approach because I think it is vital to be able to distinguish reason from justification. My mom used to tell me that there is a difference between a reason and an excuse: an excuse excuses a behavior, making it okay, while a reason explains why it happened but doesn't, in itself, justify anything. Things usually happen for one or more reasons, but often have no excuse. Most people would agree that it's important to understand the reasons that a bad thing happens - that way, you may be able to prevent it, or at least know when it is likely to happen again.
I feel the same way about basically anything that a writer does that makes her fantasy world different from the real world. The fantasy world, and the writing, will likely benefit if the writer is aware of what has to be different in her setup so that this new world order will make sense. For this reason, I'm especially interested in work-arounds that allow fantasy worlds to be free of sexism, homophobia, etc. without becoming unrealistic utopias. Because I don't want a world free of problems - that would be boring to read about - but frankly, I am sick to death of girls having to dress up as boys if they want to fight. As a fan of equality, I'd like to read more stories that have that as a basic premise, but as a fan of logic, I'd like for the equality to make sense.
Of course, figuring all this out is also an exciting opportunity to add depth and uniqueness to your worldbuilding! For example, if you want to write a fantasy world without homophobia (ooh me me, I do, I do!), you have to work backwards from some of the reasons (not excuses!) for homophobia's existence in our world, and figure out how each reason doesn't exist or doesn't cause problems in your world. For example, one issue you might encounter is confusion about how inheritance works for gay couples, especially those who stand to pass on titles and power as well as possessions. How will this be addressed in your fantasy world? If a country's queen marries the girl of her dreams, who will be the next queen or king of that country? Is there a strong adoption system? If so, how is a child chosen for such an important family? Does the child need to be a blood relation? Or maybe the rule of this country isn't inherited at all - maybe the queen came to power through combat, or was elected, or was chosen in some kind of magical selection ceremony.
Conversely, I wish many authors would look at the ways in which their worlds are similar to ours (or to their own experiences), and see whether it really makes sense for the reasons behind a certain quality of our own world to also exist in the fantasy world. Perhaps different reasons exist that cause the same effect. Or maybe the author just hasn't thought about it. I think this is the likely explanation for the many fantasy worlds in which the great majority of people are pale-skinned, often with light eyes and hair. Do they all live in worlds that are perpetually cloudy, causing them to evolve in a way that allows maximum absorption of vitamin D? Did they all evolve in one or a few such areas, then spread over the rest of the fantasy world in a conquering wave, desperate to escape the fantasy equivalent of Siberia? Is magic somehow involved in their coloration? Or is it just that the author primarily knows, interacts with, and reads about white people, and most of the characters s/he comes up with tend to be white?
Have you dealt with manipulating causation to achieve your ends logically when writing fantasy? What are some things you've changed in order to make a particular quality of your world make sense?