Poor methodology or even some bug in an Excel macro at the UN headquarters could well be a reason behind the sudden, synchronous decline of population in all cultures and political systems of this planet.

And like the article suggests it can be deliberate too. Am extremely skeptical of population figures in some parts of former Soviet Union. The official demographic loss figures in WW2 had tripled since 1945 but post-war census figures were never revised. That could easily account for the "demographic collapse" of 1990s.

Population counts are parts of geopolitics.

If you're the neighbor of some country that has a number of natural resources you'd like to get a hold of then you want to do things like formulate battle plans. If you have to make a plan to conquer 10 million people, it's going to be a bit different than one for 5 million people. The 10 million one is going to take longer. And then when you figure out that country is using deception to bolster its population numbers you have to figure where they lied about these numbers. Is it everywhere, is it in the place you want to invade. Is the population actually higher where you want to invade but lower in the rest of the country. Now you have to invest in doing your own general population and capability counts to make sure you don't step 10 feet deep in a 2 foot deep pool.