"Retard" means "thick", in this context, not "will get there eventually".

The technical definition is not how the euphemism is used.

There's an interesting etymology of "retarded". Also "idiot", "imbecile", "moron", etc.

These were clinical classifications, initially used in the early days of psychology and sometimes overlapping discredited ideas like eugenics. But these were diagnoses -- you could be determined to be an idiot, which was worse than being an imbecile, which was worse than being a moron -- by a respected doctor.

Of course, schoolyard kids got a hold of the terms and used them to disparage their (probably cognitively healthy) peers. And so with "retarded" and "disabled" etc.

But "retarded" just means "slowed or delayed". Developmentally speaking, especially when surrounded by other kids in your same age group, that's a noticeably difficult thing to be.

It does not mean (and never meant) that you are certain to reach full cognitive ability eventually. Flights that are delayed are sometimes also cancelled.