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.