I think the first part of your hypothesis is correct, but not quite the second. Most imperative languages support deep recursion and reentrancy, which means that they're a proper superset of (most of) functional languages. You may need to create a "closure" by hand and pass it around, but there's nothing that's fundamentally impossible. The example in the article about adding a side effect and cross-dependency to a deep function really isn't possible in functional terms without changing the design (i.e. adding the state to the function's interface).