Against this (purely stating my understanding of the rationale without commentary), calling genetic females simply 'women' could create the impression that trans women are somehow less 'genuine' women, in the same way that labeling White British individuals as simply 'British' while labeling BAME British as [ethnic denomination]-British would be exclusionary.
Nope. Because you're comparing different things!
British is an identity that all Brits share, regardless of skin colour. They are British because of their nationality. This is verifiable. Let's call them all cis-British.
If an American person "identifies as British," waves a Union Jack flag, and drinks tea, does this make them British? No. They are American. You can call them trans-British, if you like, but they are not actually British.
Now, within the category of British (based on nationality, the rules of which are clearly defined) you can then sub-divide. But, in order to do this you have to use a different trait altogether, like ethnic origin or religion. It would be unfair to say anyone is more or less a British national due to these unrelated characteristics.
However, British does not break down into cis-British and trans-British, because the second group isn't technically British.
Woman means adult human female.
Trans women are not, by definition, adult human females. Because if they met that definition, they wouldn't be "trans."
SO, there is no sensible, unifying definition of woman that includes trans women.
Now, you can sub-divide women by taking other characteristics into account, and categorize them by eye colour or height. That's fair. But you can't sub-divide them into women and trans women, because the second group doesn't meet the definitional criteria of the first.
If you are comparing this to race, you have to do it fairly. And bring up trans-racialism.
That would mean that you can be "African American" (actually ethnically so) or "trans-African American" (of another race but identifies as African American)
Transactivist language implies that these two groups are both equally "African American," but split into "cis" (i.e. actually African American) or trans (i.e. actually of another ethnic origin)
Does this make it clearer?