The term slavery is used in the correct way. It is not only labor rights that are denied in Xinjiang but every other right. Uyghurs essentially have no rights. Slaves have no rights. Therefore, Uyghurs in Xinjiang can be considered slaves.
Modern slavery is a fact [0]. China is not the only country concerned. But it does not mean that it does not apply to China or that we are forbidden from using the term when describing Xinjiang.
Traditionally if it's a prison the term is prison labor.
In a concentration camp you'd have forced labor. That is probably the language that would be used by groups actually working in this space.