Yes you can reduce and map a large state object into a smaller and simpler object before you pass it down to a child component but the encapsulation is still leaky because the parent component needs to know the implementation details of each action of a child component in order to use them correctly (for example, the parent component needs to know how the different actions of the child relate to each other in terms of how they affect the state); it creates a large contact area between components which creates tight coupling.
The idea of blackboxing/encapsulation is that the parent component should know as little as possible about the implementation of its child components.
Do you have a more concrete example? So far as I can see there is no reason why functional programming would require a parent component to know anything about how its child components interact with the state. The tight coupling you are describing sounds completely foreign to me as a Haskell programmer.
I think some of the spirit of encapsulation (i.e., decoupling from implementation) is achieved by polymorphic functional interfaces, e.g. type classes in Haskell.
The idea of blackboxing/encapsulation is that the parent component should know as little as possible about the implementation of its child components.