Most inflected languages have at least three genders - masculine, feminine, and neuter - and some have far more.
However, there are only two sexes, one of which is “assigned” to each person by biochemical processes at conception.
But those ‘inflected’ connotations did not, historically, include animate living people. :0) They were meant for inanimate objects.
It sound to me that they are taking the position that assigning ANY number of “genders” is “genderism,” whether it’s two, three, or 31-flavors. So, who is the “genderist” here?