What you linked to is of construction contractors building the plant, not assembly line workers. It's a blurry line and certainly affiliated with BYD, but car assembly itself is moving toward 'dark' factories, populated by robots that are indeed kept in slave-like conditions, unpaid and worked beyond humane shifts.

As a side note, counterintuitively BYD has a lot more workers per car vs a lot of their competitors, but that's because of their extreme vertical integration where most are assembling upstream parts.