Wikipedia is a wonderful online encyclopedia. It's not a source and they don't claim to be a source. It's a starting point for people who want to learn about a topic, it's not evidence of anything.
If you actually go to the sources section of that link you'll see they're all secondary sources. For some reason, nobody can get reliable evidence on what is supposedly one of the biggest human rights violations in history.
The excuse is always that China is so secretive and tightly managed that it's impossible to smuggle actual evidence out of the country. The best we can do is an endless stream of anonymous and semi-anonymous sources that keep changing their story.