It is interesting that they didn't choose "it" instead, which is the actual neutral singular pronoun. Many people already use that term to refer to a living creature whose sex is indeterminate—until they learn which sex it has, at least. So it's less weird than using a plural pronoun in a singular sense. "It" fits the NB use case exactly.

Except for the part where it implies a possible lack of personhood and therefore a lack of self, ego, etc. precisely because it leaves out the presence of a biological sex, which objects and some living non-mammals do not possess. But so does "they" unless they are forgetting that "they" is also used to refer to groups of inanimate objects.

Basically this use of "they" is simply illiterate. They want the acknowledgement of their personhood which "she" and "he" confer while pretending to be neither of those sexes.