Vit D is an important issue in Austism, and your point about the requirement for D3 in serotonin production is not to be ignored. But I don't believe it plays as a role as the paper suggests. Vagal macrophage transmission of bacteria into the brain appears to be a key component in Vitamin D receptor inactivation. until you reactivate the VDR or GC receptors the Vit D cannot do its job correctly.