Purposefully crippling your child's development is child abuse.

It's unclear if that's what's being done here, though. It sounds more like they're letting them learn at their own pace.

I know I personally went from being well behind, to almost immediately reading and writing many years above my age. Trying to really force me earlier would have probably just made me hate reading and writing, and that would have been more damaging.

To be clear, not saying these parents are doing it right, I don't know them. But it's not as clear cut as 'this is definitely child abuse.' At least in my opinion.