I think they believe that its such an impossibility, they are solely assserting their belief that 'transwomen are women'.
In thinking of transwomen, they are, as you said, thinking of those they know.
They have no idea of the wider picture and have absorbed all of their belief about how transwomen are treated / regarded by those around them.
THis is why I think it would be interesting and important to know what Maria Miller has 'seen'. Similarly the representatives from the other parties.