Language used to manipulate people.
Most people I encounter who say, "Trans women are women" are the same people who refuse to define the word "woman" or they use the word "woman" in their definition of "woman" which makes it...not a definition.
After WWII, George Orwell wrote an essay Politics and the English Language in which he wrote about how authoritarians use language to manipulate people. One of the examples that he gave was "meaningless words." He specifically talked about words like "fascism" and "democracy" which had and have become so capriciously used that they've lost any clear meaning. That's clearly happened.
Don't let it happen to the word "woman." We can afford to give any more ground.