Dawkins, Rowling, Sunak et al on Trans Issue and Women's Rights.

[The Architect Of Fate]

So, I found it on page 111 of this document. That said, somehow, I doubt that you actually looked very far into it. You probably just looked into this Human Life International page that actually uses the phrasing you quoted.

Why do I doubt you looked  too far into it? Well, that exact same page that included the cited statistics also said this: "Most of those who de-transitioned did so only temporarily: 62% of those who had de-transitioned reported that they were currently living full time in a gender different than the gender they were thought to be at birth." Which means that the rate of total, long-term detransition is closer to 3% overall.

And that's not all! The survey actually goes into the reasons they gave for doing so. And what percentage of the detransitioners do you think said "They realized that gender transition was not for them"? 5%. There's a lot of other reasons given, and the vast majority of them were due to external pressure (from friends, family, employers, what have you). All this information was on the same fucking page as the stat you cited.

So, the actual overall rate of detransition for reasons that aren't related to external pressures forcing them back into the closet? 0.4%. That's 1 in 250. And, make no mistake, those 1 in 250 deserve all the support they can get, but denying the other 249 the help they need is no solution.

And even if he's right that's a pitifully small number of detransitioners and doesn't even come close to doing what transphobes cite it for which of course is trying to invalidate trans identities.