[hkmaxpro]

Two articles explaining how different groups of observers arrive at the estimate of a million:

https://qz.com/1599393/how-researchers-estimate-1-million-uy...

https://www.chinafile.com/reporting-opinion/features/where-d...

They admit the number may not be precise (because China wouldn’t allow direct access of evidence), but it is “credible”.

This number agrees with the “detention targets” (of 10 percent of the population) shared by local officials, and the average estimates by local people.

Detention targets of local officials and estimates by local people are evidence. Not air-tight, but are reasonable when China forbids independent verification.

The second article also mentions indirect evidence, other than local reports or satellite images. All very conservative estimates give about 200,000. But these are very conservative estimates. The actual number is likely much higher, and suggests that “a million” isn’t too far off.