As an archaeologist I approve this

Cause you can’t tell gender by a skeleton? Telling sex by one is hard enough, and if we don’t understand the gender structure of the culture we’re investigating, we default to our own cultural biases.

Because they're talking about biological gender in relation to sexual organs and other traits of sexual dimorphism such as hip width, bone density, height, etc

But why isn’t it considered grave robbing?

Becase male and female as sex. Chromosomal dictated differences in the physical body. Which there's only two options. Wide or narrow pelvis when it comes to skeletal differences for example. Gender is something different that people keep changing from 2 to fluid, to spectrum, or not sure the current consensus