You are right in your last paragraph. The situation in Xinjang is such that for an Uyghur it is in fact impossible to decline to do what the state says. Any work done with involvement of Uyghur population by default is a situation of forced labour. That’s what the article says.
However, using the term “slavery” gives CCP ammunition to fight back. They can point out that Uyghurs are paid. They can claim they are free to refuse (we know they will not out of fear of retribution, but I suspect the need for such nuance only hurts the case).
Equating slavery with forced labour belittles the gravity and awfulness of what actual slavery is (or was).
Forced labour takes different forms, which includes forced paid work and by some definitions penal work.
Slavery is when a person is treated as a property of another person. Being a slave may or may not involve forced labour.
In addition, calling Uyghur work “slavery” in fact undermines the cause, allowing CCP defenders to legitimately call out “fake news”. Precision matters.
However, using the term “slavery” gives CCP ammunition to fight back. They can point out that Uyghurs are paid. They can claim they are free to refuse (we know they will not out of fear of retribution, but I suspect the need for such nuance only hurts the case).