I think that is a fairly accurate summary of s/lt's limitations in such cases Salondon. With the best will in the world, threy do not have the technical knowhow to 'jumpstart' the child with extrinsic reinforcers, which are then yoked to the sort of reinforcement we want them to respond to (eg verbal praise). They do not then know how to thin and then fade out the extrinsic reinforcers and transfer stimulus control.
They know nothing about how important, nay, vital, are aspect such as timing of delivery of reinforcement, both from task completion and from one bout of reinforcement to the other. They have not been taught these skills which are essential to engaging and moving forward a child who presents like yours.
I know, because I was one of then and worked with children like this without this knowledge for years and years. Looking back, I can only describe it as trying to walk around an unfamiliar building wearing a blindfold, gloves and earplugs. I did not have all the information I needed.
If I had a very small autistic non verbal child, knowing what I know, I would steer clear of s/lts until basic skills were established and reinforced by a behaviour analyst.
That is not to say that s/lts are not the right people for other issues. Noone comes close to them in many other areas of expertise. They have speech disorders sewn up, they really know their business in terms of feeding and swallowing and in myriad other contexts, but in this one, they are categorically not the best people to work with the child.