OP raises and interesting point.
Not having read this one groups constitution its hard to comment.
Many charities have a vague all purpose clause along the lines of "anything that furthers the purpose of the aims".
Also possible, that up until know they thought they were covered by the widely spread misinformation about accepting trans women as women (within the law).
I read both the short and long response from Rape Crisis England and Wales, which basically said each group is autonomous and they decide what they want to do.
But maybe if not now, then later, they should send members groups legal guidance on the very issue the OP raises.
Whether in the instance of the group cited, or others, we should never underestimate just how effectively Stonewall's false law had been accepted not just in the charity sector, but also businesses, as being legally correct.
On a more negative note, I wonder how many groups like this one will just ammend their aims and objectives to say women and trans women.
I suspect many will, as much because they think they are more likely to get funding it they do.
Not forgetting that funders give far more money than individuals so it may come down to that.
Sadly.