You left that part out.
>it's now controversial to ask "what is a woman?"
to Matt Walsh is pretty clear. The entire documentenary is based on the premise that it's not controversial to ask this, because the answer must inevitably be "adult human female", this excluding trans people. Meanwhile in reality, nobody finds this question controversial, only Matt Walsh's simplified extremely common sense appeal answer.
I guess if you want to be exact, I will correct to "nobody found this question controversial before it became a dogwhistle". But the question itself is still not controversial, it's all context.
It does exactly make the point, you are somehow trying to define what a woman is, and asking if the definition only encompasses adult human females or underage as well.
The beginning of this essay explains the phrase pretty well: https://www.youtube.com/watch?v=oAMM3l156Oo