I agree with you that straight people and cis people can also have confusing timelines in terms of experiences and growth and you of course don’t know what any person you meet has gone, or is going, through. Regardless of sexuality and gender.
I think the point made, the way I read it, is that because the general public still does not quite grasp the gender debate fully, there’s a tendency to think of transgender people in a very stringent way (to be transgender you must fit x, y, z standard). How can you be transgender if you didn’t know from being born? Why are you only coming out now? You’re not really transgender, etc. To be honest, similar to how gay people have been, and are being, treated too: Okay, we will “accept” you, but only if you fit a narrow definition that makes us the most comfortable (in this case a more chronological timeline to express yourself in).
I’m a genderqueer bisexual myself, just as an fyi.
Edit: I will say, however, of course you’re allowed not to like a certain writing style. Maybe this book just wasn’t for the people complaining about the lack of a chronological timeline and that’s also fine.
Elliot Page Does Not Owe You a Legible Timeline: On the Beauty of Nonlinear Queer & Trans Storytelling ( bookriot.com )