I think they probably mean intensive interaction
Research Autism currently say it has insufficient evidence to reach a view on effectiveness here
If you google it lots of info will come up.
We were told very firmly that the professionals could not assess whether my son at age 3 had LD on top of his autism (by LD I mean low intellectual function, usually described an IQ below 70), because he did not have the language or ability to follow instructions, they could not test him.
I don't want to give you false hope, but I doubt very much they can make this diagnosis at nursery age for a non compliant child with no communication, there is no way of testing his IQ until he can communicate what he knows. It used to be thought no language = low IQ but its not always the case. There are non verbal adults who write novels and films. eg google carly fleischmann.
DS has quite severe autism and had no language at 3 except for yes and no (he previously spoke in sentences and regressed). He would pull me to fridge exactly the same, almost no interaction with me otherwise.
We started ABA at 3 and he did start to speak again. It was not the words were not there he had stopped using them. It was much harder for him to learn language second time around. He in now 6 and uses phrases but not sentences and there are large gaps in his language. He still only speaks when he really wants something or is rewarded for speaking. Very occasionally he will comment on something. His language will keep improving I am sure. Its been very slow but steady progress. But he can read well above his age level, his memory is amazing he already knows his times tables. His IQ was scored at 90 after 6 months of ABA and I am sure if we tested him again it would now score much higher as he can now show what he knows.
So while you should be prepared for the worst scenario, you should not take this diagnosis as set in stone. Professionals do not know all there is to know about autism and often get it wrong.
ABA is worth looking into. It doesn't work for every child, but it has a better evidence base than intensive interaction. It is expensive and we had to fund it for a while and prove it worked before getting it on the statement.
I do think a significant number of children get put in the SLD box without any real evidence to support that so you should be wary. I remember at the diagnosis meeting the Clinical Psych told me he could not rule out SLD. All I could think was that DS was really smart because in shops he would spy the lifts and push me round a corner away from the lift, make me stay there and then leg it as quick as he could to the lift before I could grab him. I just thought how can he have SLD and be so cunning. Sometimes you have to trust your instincts. Lots of children don't make progress not because they have SLD but because they have poor quality intervention.
I don't know much about intensive interaction but I would push for interventions with a proper evidence base every time. Often LA SS use interventions which are cheap rather than ones which are effective. My son's ABA package cost 3 times the cost of a SS school place (we had to go to tribunal to get ABA), but my son has made good progress and most of the children in the SS have not.
Don't be afraid to challenge and try other things out of school, especially if you don't see progress with their interventions. If there is no progress they should try a different approach like ABA, not just assume the child can't make progress due to SLD (although obviously that is the case sometimes)