glimmer- yes I know what you are saying. I do think you would find Donna Williams stuff on this interesting - I'll look for a link a mo.
For Ds1 the biggest difference congnitively was pre and post imitation. Before the age of 8 he could not imitate. At this stage I don't believe he understood that could do the same as others. Once he learned to imitate he did - he also became much easier to teach as you could say 'do this' and show him something. Before imitation he didn't learn anything at all - zilch- for observation as he had no comprehension at all that he could do the same. It also meant he would see people having a good time, climbing, swimming, dancing and have no understanding that he could join in.
So imitation has been huge for him. but yes I think there are broadly (very broadly) two groups in ASD. Those who have the "tools" to communicate. So they have an understanding of language and they have the ability to talk. But for whatever reason they don't. I wouldn't necessarily say they choose not to - there may be initiation problems for example, but for whatever reason they are not able to use those tools successfully.
Then you have those like DS1. He is very sociable, likes people, very affectionate, enjoys social interaction and seeks it out. He likes to share interests (he's just come to fetch me to show me grandad's old care on google maps and to tell me it's broken) but he has very very limited tools with which to interact. He has no speech at all and massive gaps in language. He uses what he has very well indeed (he'll use pictures, VOCA, PECS, Photos, objects, sign, pointing, vocalising, google maps, You Tube to communicate) but it's limited by his lack of language.
And yes there are definitely concepts that require an understanding of language. Everyone who works with him says he is 'smart' and 'clever' but that does not preclude him from having severe learning disabilities (which he most definitely has). Amazing memory, but without language there is always a limit to your understanding (although you can say 'find me a green car' and he'll put the little google map man down right next to a green car).