Orders of magnitude. While both strain credulity, it is more plausible to take a government at its word for 300 people than for 3 million.
Plausibly it could be the case that there are 100 wrongly imprisoned people in Camp X-Ray without legal recourse... But equally plausibly there are at the very least 1,000,000 wrongly imprisoned people in the Uyghurstan concentration camps, with thousands added every day...
Orders of magnitudes in difference.
Holding a few dozen suspected terrorists !== holding 3 million people (nearly an entire race)
You can justify there may be proof for those few alleged jihads. You cannot reasonably prove those 3 million people are all terrorists.
Where are the cries to shut down these camps like there were cries to shut down Gitmo?