It never says that the crazy describes all trans people. It doesn't call all trans fakes. You seem to even admit you didn't make it past the title with your uncharitable read, so how do you expect to respond to something it actually claims?
It points out that there are some social contagion factors to some expressions in our youth. Which is completely reasonable given how much clout gives you in modern social media, for example. And that an impressionable girl with low self-esteem can suddenly "become trans" and win social favor, which I don't see how you could find unreasonable if you've ever been a teen.
Maybe approach it a little more open-mindedly. You basically dismiss everyone in the book as just being unable to "cope with loss" which isn't much different than dismissing any group (like trans people) with some convenient reason that leaves your world-view untouched.
Also not sure how your anecdote disproves the craze, especially since the craze is talking about the last 5 years. Your experience in the 90s has little bearing on 2020 where you get instant social proof on social media in some circles by putting a rainbow flag in your username. Times are different. You don't think social contagion could explain any part of the 800% increase in trans identity among the youth? I don't see how you couldn't believe that on any 800% increase in what the youth are doing.
It's like amphetamine abuse among kids. I have extreme ADHD and to some degree depend on the drug to get what I want in life. Just because you could write a book about kids claiming to have ADHD just to get prescribed a great recreational drug (and I bet you could even call it a craze, as it seemed in uni) doesn't mean I can't have ADHD or that everyone is an abuser. I would consider myself weirdly defensive to the point of dismissing inconvenient truths if I were to believe that, yet that's what I see you doing.
That you give yourself so much authority on trans matters, especially in response to such a reasonable point, is kind of an example of what I see as modern trans clout these days. Why wouldn't a teen want that? As the book points out, it's incredibly easy to "be trans" when the rest of your friend group in high school is suddenly on the non-binary spectrum unlike when you were a kid.