Just look at it backwards. Even before the 1 child policy, both china and india have had a gender imbalance favouring males. After the 1 child policy in china alone, both india and china had a strong growth in gender imbalance, with china being only somewhat stronger. Your "control" without the 1 child policy also exhibited the same strong growth. A scientist would reasonably conclude that the 1 child policy seems to have little relevance to the issue, considering that another comparable country behaved in exactly the same manner without have such a policy in place.
Both China and India share pretty much exactly the same imbalance until the last two age-cohorts as you suggest, but while both increase, China's does so substantially more. This can't be ignored as being "exactly the same manner". To do so is being scientifically irresponsible.
My take is that both cultures have very prominent views on the preference for male children. You can see this even in the large Chinese and Indian communities here in North America. What has most likely happened is that the one child policy has simply enabled those cultural preferences in China much more than the exist in India.
It's also interesting that India itself has at several times flirted with restrictive population polices, including forced sterilization at right around the same time as China introduced their family planning program.
While that doesn't suggest that the one child policies are the only cause, I think there is enough evidence there to suggest they are a factor.
Nonetheless, all of this wasn't actually a real focus of my original point at all.
