I wondered if anyone else has a child who has adopted this approach as a survival strategy in the face of incomprehensible language and social rules.
What DS2 does around other children is shadow DS1 (2.8 years older, similar interests) when he wants to join in a game but can't understand what's going on. He literally stands just behind DS1, waits for DS1 to take a turn jumping/running/skidding/saying something silly or whatever the game is and then takes his own turn by copying Ds1 as precisely as he can.
When he was two and three, DS2 used to do this every time he wanted to enter into kids' games - he did it today when he couldn't follow a conversation and it reminded me that this is how he has learnt to be a child. It's a sort of intensive early play therapy, I suppose .
When it comes to non-physical social play without fixed rules though, he is much more at ease with a typical 3 year old than a 4 year old. He and other train-made boys kind of talk across each other about what trains they have like blokes in a pub talking about cars.
I used to think that his adoption of this strategy meant he "didn't have ASD" but I've kind of got beyond the whole is/isn't thing now......