That is literally what the article describes, though, in Papua New Guinea. And it describes why states in Nigeria have such a strong incentive to fake their population numbers, that it's impossible to achieve an accurate national total.
I do think the headline exaggerates, I doubt "a lot" are fake, but some do seem to be.
If you pick any country and look at proxies that have significant cost associated with them, at relative population levels of verified locations, the population of the world differs pretty radically from the claims most countries put out.
If you don't have independent verification free from censorial pressures and legal repercussions, then you get propaganda. This is human nature, whether it stems from abuse of power or wanting to tell a story that's aspirational or from blatant incompetence or corruption.
Population numbers fall under the "lies, damned lies, and statistics" umbrella.
Yes, they get their data from each country.
How else could they realistically get that information?
Isn't this normal and understood?
Since the data is only possible from the government of the country and you believe it's fraudulent there's no legitimate source of information.
since people may need that information and there's only the single source what's the issue with WHO?
You also claim that anytime a country provides data , the country believes they will benefit if the data has a some value, and they can't get caught then they will lie.
Shouldn't you just be suspicious of any data like that and investigate?