This number is misleading, if not downright wrong. And, as a result, you will notice Rowling has no basis for supporting it with facts. From what I understand, it is true that most children who express gender identities inconsistent with the sex assigned at birth, or who express gender in an atypical way (such as girls who are “tomboys”) eventually identify as the gender they were assigned at birth by the time they are in their teens. However, teenagers who actually display dysphoria (or even children whose identity issues are persistent over long periods of time) claim the identity of the birth-assigned gender much less frequently. A girl who plays with GI Joes at age 7 is not nearly the same as a 7 year old trans boy who has been asserting that identity since age 4 and has been treated by doctors and therapists specializing in gender identity.
One more thing: I’m pretty sure she got that number from a study that has since been discredited, and that the number quoted is a mischaracterization even of the discredited work. I don’t remember the name of the researcher (I want to say McHugh, but not sure), but it is one of those who think trans women are really gay men and that trans men don’t really exist, if I recall correctly.