how does one know what a woman feels like when you were born male (and vice versa), it strikes me as a being an absolute impossibility

It is an impossibility. For one to be able to make such a claim they would need to be able to form a comparison between what it feels like to be a man vs. what it feels like to be a woman, and of course there is no possible frame of reference that exists to make that comparison as we each only have one body.