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I believe there are cases of people being born with a set of sex chromosomes that are different from XY and XX. This is what your parent commenter is referring to and I fail to see how it's a valid point ("anti-science"), on the particular topic of determining biological sex in people.

If one wants to make a simple statement that encompasses the largest group of people they might say, "There are 2 sexes." If they want to make a more complex statement that encompasses all humans they might add a qualifier such that they instead say, "There are 2 typical sexes." That "typical" adds a whole lot of complexity in order to generalize the statement.