What is it ???
The short answer is ScrollView is a Layout. But it can only contain a single Visual Element in its Content property.
Why So ???
Because, the Layouts such as Stack, relative etc. are derived from Layout<T> generic class. This Layout<T> generic class is derived from the Layout class, which is a non-generic class.
ScrollView is inherited from the Non-generic Layout class. That is the same reason, why it does not support a generic way to add children. It only exhibits the Content property to accept a single view or a Layout.