Why is it bad to say trans women aren't the same as biological women?

I watched a video about JK Rowling today, and the person in the video said that JK Rowling's tweets were very harmful and were making life dangerous for trans people.

As far as I know, JK Rowling has taken the stance that trans women, while still women, are not the same as people who were born as women (I hope that's the right way to say that). But isn't that true? At a biological level they are different, that's just a fact. Why do people get upset about that, and why is that information dangerous for trans people?

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