Well, China has been a fixture of American politics, something to be feared ( as was Japan in the 80's, political fervor over their economic success, followed by 2+ decades of a flat Japanese stock market - so Japanese "concern" as baseless and only served political ends ). So do we American citizens write off the China talk as only political and ignore the chat about it or do we take China seriously as a threat, et al? Knowing more about what China is can assist with how I relate to China politically.
Well, this seems like an interesting question, epistemologically. If we take your example (which, mind you, I am ignorant to the facts of, frankly) as, in fact, being the case, I guess we could question what the role of what we did knew of Japan did to lead us to the, apparently, unfounded fear? I'd think (but not know) that we had "better" or more information from Japan, given it was not nearly as "closed" as China is.
Unless it is the case that we didn't know what we thought we knew (that is, we were mistaken in the facts), or that we did know facts but we did not understand the implications of those facts. Or, did those fact indeed lead to action that made the initial forecast appear false? The question then, it seems then is would precise facts about China lead to a different outcome?
So, in a sense, I'm not sure at all. If we knew China's exact GDP or population, for example, does that mean that we have what we would need to know exactly how the future plays out? Or, what intervention is needed or unneeded? It's unclear to me. I would think, just off the top of my head, that it would be more crucial to know what China is doing than precisely what state China is in. In other words, what policies are are undertaking, what action are the taking and so on.
Perhaps though this is not pragmatic for some reason I can't think of at the moment. Still, it seems much more feasible to be able to "see" that, rather than thinking we (or even China itself) could know just how many people are in the country. Or maybe I am just being overly skeptical for no reason.