Wouldn't your causal mechanism be disproven by the reams of peaceful countries without such imbalances? In any case, the natural overproduction of males versus females at birth is too tiny to explain these gulfs.
As I understand it, the grandparent's argument is that earlier on in history, wars (and violence in general) would cause a reduction in the percentage of males. For example[0], in Soviet Russia following WW2, the ratio of young men/women dropped to 0.7. And that given that the China and India are more peaceful now (for various reasons), this violent force does not come into effect.
I don't understand how your response is relevant to the above argument.
Peaceful countries with balanced population has better woman rights. What China/India has, isn’t really natural because they tend to abort female babies.
I was just today reading a piece arguing that measures showing historic lows in war deaths are wildly misleading because so much war today takes the form of civil war, rather than inter-state conflict, something which the measures don't cover very well.
At birth, there is a slight bias towards males born. However that is something in the order of 51% male babies to 49% female babies. That is far less pronounced than what is being observed.
That's not too far off from the figures quoted in the article – a 2% difference of 1.4 billion (China's population) would be 28 million, which is not too much less than the 34 million difference quoted in the article.