I would think that the fact that people disagree on whether the Earth is flat implies that it's not clear if the Earth is flat.

I am not taking a stance on gender numbers, but that exact argument is being used against climate change and vaccine safety.

Gender (ascribed or identity) is a social (ascribed) or subjective psychological (identity) construct, and biological sex is a matter of arbitrary line drawing across a field with multiple axes of variation. The number of any of the three that exist, or should be recognized as existing is not the same type of question as vaccine safety or climate change, and arguments that are perfectly valid for one of those types of questions are not for the other.