ThinkWittyThoughts
I think that it is slightly more complicated than that - not that I want to defend the GIDS. However, my understanding is this: the 80% figure comes from old studies from when "transkids" were tiny handful of children, mostly boys, who were dysphoric from a very young age. Most of them desisted moderately early in puberty and clinicians like Zucker said that if the dysphoria had persisted well into teenage years, they'd probably be among the 20% who persisted permanently.
All that has been thrown into disarray by the new phenomenon which is mostly girls, very large numbers, and the dysphoria actually starts in puberty. People who aren't braindead see that it has all the hallmarks of a cult and think that many of them will desist or detransition. Sensible clinicians like Zucker say that the same rules don't apply to this cohort. However, nobody really has any proper empirical evidence on what will happen to them, because it's a new thing and because nobody has even been studying or keeping track of them.