No, you misunderstand. (You're probably not alone.) Hawkins knew fine well what a woman is.
Hawking said something and you knew what he meant. For this to be true, you both need to know what the words he used meant.
You know what 'Women are a mystery' means. So you know what "women" means. So you know what is or isn't a woman. As did Hawking.
Hawkins didn't say anything like, "The meaning of (the word) 'women' is a mystery," did he?
(Is this an example of so-called "use/mention" confusion? Look it up if you're interested.)
Knowing what words mean is important for understanding everyday discourse as well as in law. Trans rights activists at first managed to confuse themselves over the meaning of words such as "woman", then tried, with some success (!), to confuse others.
Don't be confused. Use your common sense. A transwoman is a man, a transman is a woman. When we talk of men - in everyday terms as well as in law - we include those men who are transwomen. (TWAW can't be true... Why? Because a man can't be a woman.)
We always knew this. It's just common sense. That it needed clarifying at the Supreme Court is a bit gobsmacking, given how clear it all is. But there you go. We do live in strange times.