I may be mis-interpreting this but my take on the abstract is this:-
that babies born with an older sibling who has a confirmed diagnosis of ASD will have a higher likelihood of developing ASD (due to genetic factors).
if the new infant has this (genetic) propensity, early intervention on social reciprocity with the parents will help the child at a very early stage before the infant could possibly be assessed for a diagnosis
that investigating what the 'best practice' in early parent-child interactions is, will help develop guidelines for parents who have one child with confirmed ASD.
It seems the same as what I have learned from SALT's Hanen courses where I have learned how to help my ASD child learn social skills with his peers- just at a much earlier stage.