It's more midrashic kabbalism which says that Adam, whom they call Adam Kadmon, was an androgyne and that God originally made man as an adrogynous being (Gen2:21-22). They justify this by so interpreting Genesis 5:2, which is likely the ultimate source of their They/Them pronouns, though those who fall for this wouldn't necessarily be able to trace it back to that and likely as not have no clue as to the true depth of this concept beyond just their own selves amidst their psychologically and societally induced dysphoria