Praise be to Allah.
The hadeeth came after the revelation of the hijaab and after it was made obligatory for the believing women. But the full hijaab is only for free women; slaves and concubines should not resemble free women by wearing the full hijaab. A slave woman does not have to cover her face, and ‘Umar (may Allah be pleased with him) used to forbid them to do so. This is the case if there is no fear of fitnah from them; but if there is fitnah, then they have to do whatever will prevent that fitnah.