It's not a basic biological fact, because biology isn't that basic. It gets really complicated, and there's new research happening all the time, including about trans people.

For instance, did you know there are multiple biological differences between a trans woman and a cis man? Certain brain structures in a trans woman more closely resemble cis women than cis men, and that's even before any kind of transition.

Also, trans women will often have surgery and take HRT to have the same hormones as a cis woman in order to change secondary sex characteristics. Really, one of the few biological changes we can't make is to chromosomes themselves.

And even ignoring all that, being trans often results in gender dysphoria, which is a mental condition. Imagine someone says they have bipolar disorder, and your response being "yeah, and you'll always have bipolar disorder. That's just a fact, get over it."

I don't think that's exactly what you're doing, but there's a way to talk about the realities of being trans that also shows you are trying to be supportive and not dismissing someone's struggle with their identity. And that's why JK Rowling gets hate, because she tends to be on the dismissive, terf-y, 'you aren't real women and never will be, get over it' side