This quote from the source you linked seems to suggest an obvious reason why it might be a controversial "diagnosis":
> Lisa Littman, at the time an adjunct assistant professor at the Icahn School of Medicine at Mount Sinai, coined the term rapid-onset gender dysphoria in a 2018 study based on an online survey of parents on three anti-trans websites who believed that their teenage children had suddenly manifested symptoms of gender dysphoria and begun identifying as transgender simultaneously with other children in their peer group.
Doesn't sound terribly scientific. I do look forward to seeing a robust conclusion drawn from studies over the coming years, though, so we can find out how real it is!
> Lisa Littman, at the time an adjunct assistant professor at the Icahn School of Medicine at Mount Sinai, coined the term rapid-onset gender dysphoria in a 2018 study based on an online survey of parents on three anti-trans websites who believed that their teenage children had suddenly manifested symptoms of gender dysphoria and begun identifying as transgender simultaneously with other children in their peer group.
Doesn't sound terribly scientific. I do look forward to seeing a robust conclusion drawn from studies over the coming years, though, so we can find out how real it is!