So was Hari's exile to Terminus, which IIRC wasn't mentioned in the original stories that later formed the 1950's trilogy. That first part of the first book was written a few years after the original 8 shorts and IIRC contained things not at all alluded to in those shorts.
It's in the book series though. So they did the rather sensible thing of not doing the sudden reveal, but slowly building the anticipation and understanding that there are people with immense psychic powers etc.
I mean, in the book series there's even the out-of-the-blue Mule without any retconning.