Another way to say this could be if you decide to take the route of explicitly not solving an edge case or write throwaway code, that feature becomes a leaf until the branch of immature code can be pruned and a new, stronger one put in place.
It doesn't become a leaf automatically. You have to force it to stay a leaf. If you don't find a way to do this, and if things grow on top of it, then you are in trouble. This is hard to do in most architecture patterns.