Sigh.
It's about recognising that female people exist and have experiences, challenges and risks that are sometimes different to male people. And that is true of the workplace just as much as anywhere else.
If you make it a "women including transwoman" group, you are effectively taking a big black marker and scribbling off the agenda any discission or support for the lived expereinces, challenges and concerns of female people. You are turning your "woman's" group into a group whose area of commonality is something hand-wavey to do with "feeling like a woman in your mind".
Now, I'm not saying that people of both sexes who do "feel like a woman in their mind" should not be allowed to have a group that focuses on the consequences of having that feeling at work.
But I don't think it should be at the cost of female people not getting to have a group that can focus on us and our needs.
I personally find it highly demeaning and offensive to be told that male people who believe they feel "like women" need to be treated as if that feeling actually makes them meaningfully more like me than other man. I think it imposes a reductive and sexist idea of womanhood on to me.
And the idea that my emploer might be the one imposing that identity onto me is just horrible. It would make me feel that my employer just saw me in terms of sexist sterotypes.
So no, it's not about "revelling in exclusionism", it's about the right of women ike me to not be redefined into a nebulous gender identity. It's about our right to be able to say clearly how we see ourselves rather than having some weird assumed commonality between our bodies and some men's feelings imposed upon us.