I'm not sure if you really don't understand what you just agreed to. You initially claimed that it doesn't matter what "women" means when you ask, "Do you sincerely believe that a person becomes a woman simply by self-declaring it?"
With a reductio ad absurdum disproof, you've now shown your claim to be false and agreed that the definition of "woman" matters. So if you're still interested in the answer to your question, you may now provide your definition of the word "woman."
> You initially claimed that it doesn't matter what "women" means when you ask, "Do you sincerely believe that a person becomes a woman simply by self-declaring it?"
No, I said it doesn't matter to to the point I am making what I believe, except that I reject that particular definition, because it is absurd on its face. I may or may not reject any number of other definitions of "woman", it's besides the point.
As an analogy (futile, I know): There are many definitions of what a "doctor" could be. For example, I can accept the definition that "a doctor is someone has been licensed to practice medicine". I can't accept the definition that "a doctor is anyone who self-identifies as a doctor".
Of course, according to the definition that "a doctor is anyone who self-identifies a doctor", someone who self-identifies as a doctor is a doctor. It's obviously true and obviously absurd.