What they did was they interviewed eight ethnic Uyghurs from eight different villages in Kashgar Prefecture in Xinjiang (the poorest and most problematic area in terms of violence and extremist ideology). They asked them questions like “how many people do you estimate live in your village” and “how many people do you think were taken from your village”. They then calculated the ratio of people detained to total inhabitants in these eight villages and extrapolated this ratio to the entire Uyghur population in the province of Xinjiang (population 23 million, many parts of which are prosperous and violence-free) in order to obtain the million+ estimate. The 1 million number (or 3 million, it’s hard to keep track of these days) is always thrown around like it’s a confirmed fact, which it decidedly is not.
Furthermore, as the article mentions, CHRD is a group based in Washington, D.C that is almost wholly funded by U.S government grants [2, p.9]. There is a clear conflict of interest here especially in the context of growing rivalry and geopolitical posturing between China and the U.S. The article also mentions Radio Free Asia, an organization literally founded by the CIA [3][4] as an anti-Communist propaganda arm. They claim they are no longer affiliated. Again, another source wholly funded by the U.S government.
[1] https://www.nchrd.org/2018/08/china-massive-numbers-of-uyghu...
[2] https://pdf.guidestar.org/PDF_Images/2015/260/523/2015-26052...
[3] https://en.wikipedia.org/wiki/Radio_Free_Asia
[4] https://www.nytimes.com/1977/12/26/archives/worldwide-propag...