It is not.
Lisa Littmans research behind „rapid onset gender dysphoria” is a survey amongst parents recruited on three anti-trans internet sites and communities:
https://psychcentral.com/lib/there-is-no-evidence-that-rapid...
The study was based on 256 responses to an online survey of parents recruited from these three websites
That by itself means its heavily biased research on a weak sample.
„Stats for Gender” site is ran by Genspect, which is also a biased source on the subject: