You're welcome EndlessLight, there may be even better explanations around now.
As for helping her grandchildren, the OP would be well advised to find out as much about autism as she possibly can, because it comes across as her wanting to force her granddaughter into appearing to be NT because she is ashamed and sees the reality as her granddaughter being slow or behind or in some way not as good as her peers, despite trying her hardest.
Many neurodiverse children are emotionally around two thirds of their chronological age. Granny thinks "age appropriate activities" should be encouraged. Granny needs to look at her own motives for this, is it because she doesn't want to see her granddaughter as immature or different to her peers and have her friends comment on it, or should her focus be on granddaughter should what makes her grandchild feel happy and secure.
Activities, some NDchildren thrive on a lot, others live in a world that's too bright, too loud, too extreme for them and they struggle to tune out background noise so they struggle to follow what's going on. They need time to regain their equilibrium, where they feel safe and secure and are not bombarded with the everyday stimuli the rest of us take for granted and filter out.
Sport is important for everyone, but if the ND child is dyspraxic or has hypotonia, EDS etc. team sports aren't a great choice. Individual sports are much better like swimming, horse riding, archery, athletics, it all depends on the child's physical capabilities on top of their autism.
Many ND children find unstructured socialisation too stressful, especially in groups where things don't follow a certain order. Many need the reassurance of now, next and then to deal with anxiety. Many are oblivious to hints and inference.
I'm sure the parents know what they are doing and know how to help their daughter be the best she can be, in her own time.
To outsiders, including granny, the way they parent their child may seem odd or "wrong" but that's because a neurodiverse child needs to be parented very differently to an NT one.
The parents may not wish to divulge their dd's diagnosis and interventions to anyone else, it is of course confidential medical information.
I hope that tiny insight has helped people to understand that by thinking " by her age she should be able to do xyz " they will stop in their tracks and think " By that age, an NT child should be able to do xyz, but as this is an ND child, they may not be able to do xyz at the same time as their age peers."
Neurodiversity is not wrong, it's just different.