WTF do you think trans people think about that idea [that transwomen were "born male"]?
I think that some transwomen would recognise that as a biological fact, and others would dispute it, because they unfortunately are suffering from a delusion. And others would dispute it because they don't understand the most basic facts of biology. And still others would dispute it because it suits their political agenda to dispute it. Different people would respond to that statement in different ways, what a shock. Their reactions wouldn't stop it from being anything more or less than a biological fact.
A person born with a Y chromosome, male genitalia and all other biological features of maleness, who genuinely believes himself to be of the female sex and to have been born female, has a delusion. From Oxford: Delusion (n): An idiosyncratic belief or impression maintained despite being contradicted by reality or rational argument, typically as a symptom of mental disorder.
To say this isn't unkind, or hateful, or transphobic, or bigoted. To label these facts with those words is to buy into the idea that there is something shameful about having a mental disorder. There isn't. There shouldn't be a stigma about it, and people who consider themselves to be transgender shouldn't be discriminated against any more than people with anorexia should be. Or schizophrenia, or biopolar disorder, or dissociative personality disorder. They should be full and equal members of society and treated with respect and compassion. They should have access to reasonable accommodation like anyone else who has a disorder or disability. Maybe this means access private bathrooms or changing rooms. Maybe it means that if there's a women's support group set up by a university, there must also be equal accommodation (the use of a room, a budget if applicable) made for a trans support group, if there are trans people who wish to establish one.
But what it doesn't mean, is that the rest of society has to throw away reality and rational argument, and pretend that delusion is fact. And it doesn't mean that people who were born female should be any less safe, or should be made to feel any less safe, or should be made to feel any less comfortable in female spaces, or should be less accurately represented in records of achievements or crimes or pay rates or any other statistic that means something to women, or should be hindered in their discussions and activism on issues important to women.
I am very close to and supportive of my schizophrenic relative, and I have an enormous about of love and respect and admiration for him. But I don't pretend for his benefit that he's absolutely right about the house being bugged by the CIA, and I don't feel compelled to live my life as if that were true, whispering into the ears of the rest of the family, out of respect for someone else's delusion.