This theory leads to some strange conclusions. If the only thing that makes someone a woman is their gender identity, but gender identity is not observable in female women there is *nothing* you can learn about the nature of womanhood from studying female people....
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...but that drawing any conclusions about the biological reality of the large group "female people" from studying a much smaller group which does not overlap with it "male people who want to be treated as if they were female people" is unjustified
But what can you study about the nature of womanhood in females if the *only* thing that makes them women is their innate gender identity, and that is unobservable -- the only indication that exists is that they don't declare to be men