it undermines those who really were "assigned." i remember in college reading about how many interstitial sex babies there were, it was astonishingly common (we are talking 1 in 2000 type common, which is quite common really). in some cases there was intervention to sort of steer the child biologically toward a certain gender (including surgery), and i would consider that "assigned" gender. but the majority of babies are not "assigned" a gender in any medical or biological sense, and to posit that they are undermines the significant minority that does have that experience.