This analogy seems forced to me, but it got me thinking.
The article says:
Nobody sensible thinks that the existence of adoptive parents undermines our understanding of what it is to be a parent. On the contrary, it extends it.
By implication, the existence of transwomen 'extends' our understanding of what it is to be a woman.
I feel the analogy doesn't work for two reasons.
Firstly, 'parent' is also a verb, and parenting is a role and responsibility. 'Womaning' on the other hand isn't a defined role. There isn't a verb 'to woman', meaning wearing make-up and feminine clothes, working in a low-paid, caring job and deferring to men. 'Woman' is a physical reality - it means adult human female. Anyone who thinks 'woman' is a distinct social role is probably a conservative and not a feminist.
Secondly, 'parent' is a relationship. It's defined entirely in relation to others. 'Woman' is not, except perhaps in the most conservative societies. I know there's a theory that the word 'woman' comes from 'wife of man', but having Googled it that seems to be a misunderstanding.
If the assumption behind the adoptive parent analogy is that 'woman' is a role and a relationship, this speaks volumes for the traditionalist views behind trans ideology.