It does? It's just the way that the interfaces are setup in the api, they don't allow child interfaces for child interfaces, or at least a way to get the id of the child interface, and since the inventory on the trade screen is part of a child, that is on the trade screen, we cannot get the items for it.
Example
Parent 0 (trade screen)
-child 1 (username)
-child/parent 2 (Left side container)
--child 0 (Items)
No way for us to access the (items), since you RS2InterfaceChild doesn't support children.
Correct me if im wrong.
According to @Merccy it actually required a new hook. He's working on it now.