you are right. actually, the footnote is misleading because of its terseness. i will modify it. the child of a slave follows its mother (in its state - mukatab, mudabbar) if it is by a husband other than the slavegirl's master; and becomes free upon the death of the master. and if the child belongs to the master, it is free by birth. since the context was about umm-walad and "gives birth to her mistress/master", the footnote was written with that in mind.