You seem determined to be obtuse or play Devil's advocate.
This is my opinion based on what I see: of course sex and gender are related - gender traits are born of the two sexes - but that does not make them the same.
A person has a sex: to variable extents, each woman or man displays the gender traits related to that sex. If you are comfortable with your sex, your gender becomes irrelevant as it extends from it. Some people identify as a different gender; they have broken from their sexually imparted traits. They cannot change their sex - all their cells will continue to hold the blueprint of XY or XX (ignoring the small percentage of XXX, XYY and XXY people out there), but they can alter parts of their bodies to reflect the visible norm of the gender they choose to be, such as breast enhancement/ removal, or alterations to genitalia, hair and voice.
The result being that gender is a projection; you can call that 'pretend' if you like - personally I think that to the person living the different gender that will be more 'real' for them than their sex - but sex is scientific and unalterable.
My opinion of this article is that this mother is right to feel concerned about anyone aggressively promoting a life-changing decision. I can see that the experience of some trans people could give rise to a militant promotion and encouragement of 'rights' the moment a child starts questioning their gender identity. A change like that should be slow, measured and organic, not like a fast-moving stream that someone steps into that takes them over the waterfall before they know what's happening. I also think that it would be good if counselling between children and parents could take place (it may well do already) - not so as to persuade but to simply state how it feels for each. Ideally, this should be met with acceptance and empathy on both sides, but ultimately, if the child decides to go forward with a new gender identity then the parent should support this or risk long term harm to, or alienation from, their child.
It is my opinion that incidents of gaslighting, or aggressive promotion of a new gender identity as the mother in the article describes, would not now be happening if the parents of those first transitioners had unconditionally loved the person inside and not the physical shell they gave birth to, and society in general had been more accepting of someone with a different gender identity. Yes I'm sure it can be tough for some, but as someone with a trans person in my family, I can assure you that love and acceptance go a very long way to diffusing any tensions that might arise. We all remember the person before, but we don't grieve the loss of them, and we certainly don't see a barrier come down across our 'shared story'. It goes on, but the characters are a little different. In writing this article, this mother is ultimately fanning the flames of a fire that's destroyed her vision of life with her child. If she changed her vision, she would still have a beautiful relationship and life with her child.