There's a conflation between sex and gender. In the article the proponents of this non-binary classifier say that "sex and gender should not be assumed" and then go on to only argue how gender is fluid and completely neglect sex.
Point being, I don't think archaeologists really attempt to identify gender, by this modern 'fluid' definition, and only really identify sex, which is a strictly biological variable. Identifying 'sex' is as objective as identifying the species, there's no logical allowance for fluidity.