Yeah. The linked source is upfront about that, but its the closest thing we have to a real study sadly. As I said, the averages are close anyhow.
Scholars have from time to time thrown their careers away by trying to get better numbers, inevitably some group doesn't like the outcome and they become embroiled in endless debate while their career implodes. For example, the major sources cited in The Bell Curve have had their titles stripped and been hounded to the ends of the earth.
All these years later people are still specifically authoring papers to debunk their work.
https://www.sciencedirect.com/science/article/abs/pii/S01602...
We will never see real numbers. This, or other things like it, are literally the best it will ever get unless someone sacrifices their career, and maybe their own safety, to gather better data.