Yes it is legitimate - it is posted on the NEJM website. An editorial advocating for the removal of gender on birth certificates.
Designating sex as male or female on birth certificates suggests that sex is simple and binary when, biologically, it is not. Sex is a function of multiple biologic processes with many resultant combinations. […]
The biologic processes responsible for sex are incompletely defined, and there is no universally accepted test for determining sex.
Assigning sex at birth also doesn’t capture the diversity of people’s experiences. About 6 in 1000 people identify as transgender, meaning that their gender identity doesn’t match the sex they were assigned at birth. Others are nonbinary, meaning they don’t exclusively identify as a man or a woman, or gender nonconforming, meaning their behavior or appearance doesn’t align with social expectations for their assigned sex.
Sex designations on birth certificates offer no clinical utility; they serve only legal — not medical — goals.