They claim the accusation that "it assumes the truth of the proposition that it sets out to prove, namely, that sex and gender are simplistically binary" is false - one of their abstracts even makes explicit distinction between sex and gender [1]. While technically even biological sex is not entirely binary, it is, in the vast majority of cases, binary enough to support categorization of human remains as male or female.
Perhaps these points would have been made in their talk, however, it was shut down without notice to its presenters, despite having earlier gotten approval for it.
[1] In “No bones about it: skeletons are binary; people may not be”, Elizabeth Weiss wrote: “In forensics, however, anthropologists should be (and are) working on ways to ensure that skeletal finds are identified by both biological sex and their gender identity, which is essential due to the current rise in transitioning individuals.”