Gotcha. Their point is kind of right though; sex is less of a strict binary category and more 2 clusters we (people) created that allow us to more easily classify specimens based on strongly correlated traits. Both clusters have some overlap, and no trait on its own completely determines the cluster.

E.g. I knew a case of this woman who grew up her whole life never knowing she has XY chromosomes, because she had seemingly typical female sex characteristics. It was only when she and her husband where struggling to conceive and they went to a fertility clinic, that that fact came to light. “Biological male” might be the cluster you’d want to put her under, but she lacks many of the features of that cluster, so in that case the binary classification is a little weak.

Of course most people/animals are not intersex (or transitioned), but the point is that the biological sex binary is kind of a shorthand / way of making life easier to classify most of the population, but it’s not perfect or tidy.

The easiest way to stay accurate is to just narrow down to the specific relevant trait (“person with facial hair”, “person with androgenetic alopecia”, etc.) depending on what specifically is measured/being talked about. But being that precise can come at the expense of being less clear/accessible to the layman, which is why we use biological sex as a concept.