This is imagining an API to understand other programs. These programs, either large or small, are called "thunks" here. This isn't exactly what a CS thunk means, but it's also not not what it means, so it seems like an OK word.
This is also related to Dataflow programming, where the containing environment manages these thunks and the flow of data between them.
Thunk static API
A thunk is typically some source code (though it could be something besides string code), but its internals are specific to that specific kind of code. From the outside there's a more normal representation: