The main reason for this guideline is that an adopted child (particularly an older one) can be expected to come with issues, may well be violent and almost certainly extremely time consuming. They will need massive input of time and attention to bond with their new parents and to deal with any previous trauma- even if it is only the separation trauma resulting from having lost their biological parents (and remember, many adopted children have been through much, much worse than this).
This can be difficult to cope with for parents who also have to meet the demands of a younger child; and it can also be difficult to keep a younger child safe if the adopted child has angry periods.
Adopted children often need to be the baby of the family because they are the most vulnerable. They often regress and go through the baby stage at a later age, because that's when they feel safe to do that. And once they feel safe, they may well start to act out their hidden aggressions. They cannot be expected to understand and meet the needs of a younger, more vulnerable younger sibling.
Don't get me wrong, adoption can be a great thing. I have an absolutely gorgeous adopted brother, whom I have loved dearly from the moment I saw him; I can't imagine life without him. But there is no doubt that he did kick the hell out of me at various points of our childhood despite my being 3 years older and a lot bigger. No problem, because I was old enough to handle it and feel maternal towards him- but would I have done that if I had been younger than him? And he needed time from both our parents in a way I never did: doubtful how I could have coped with that if he had been older than me. Yet as far as I know he had never suffered any of the really horrible things (abuse, drug exposure etc) that so many adopted children have.
Not saying you can't do it. But I think what you do have to ask yourself is: given the needs of my autistic son, would we be able to deal with a period (maybe of several years, maybe recurring throughout their childhoods) when the adopted child needs to be the baby and the one that gets most time and attention? And if your ds cannot cope with younger children, how well would he cope with an older child who had the needs of a younger child?