GDI is not people's wages which is what the author mistakes. And GDI is not GDP because trade factors need to be netted out.

Directly from your link:

"Because the two series are measured differently"

I forgot that two correlated series means they're the same thing. Huh

> In principle, these two series measure the same object.

Measurements have errors, particularly with something as complicated as the economy. The series are correlated because they are different measurements of the same underlying quantity. The correlation is not perfect because they measure that quantity differently, and so have different errors.

There being 3 different ways of measuring GDP (income, expendeture, and production) is not controversial. I am struggling to find any source on GDP measuring methodologies that does not acknowledge all 3.

Put another way, what produced value does not end up as income on someone's balance sheet; and an expense on someone else's?

Yes, a nation's economy is not a closed system, so you need to account for imports and exports, and this accounting manifests differently depending on which methedology you use.