No, its also based on the fact that older parents (and especially fathers) lead to more de-novo mutations (which you can already see in sperm) and in this study

No, its also based on the fact that older parents (and especially fathers) lead to more de-novo mutations (which you can already see in sperm) and in this study now they show that especially de-novo mutations play a role. If the older parents hypothesis was not true, you would not find higher autism rate is younger siblings or how do you account for that in your alternative explanation? But sure, as they say in the paper, its most likely not a single large effect variant that causes autism but usually the combination of multiple rare variants. The genetically inherited part may also play a rule. For a complex disease such as this, its not a single cause that is responsible even for one patient. A tall basketball player can be tall due to some luck, tall parents and good nutrition all at the same time.