Your assessment is correct: fill-in-the-blank currently is not supported as a task only as an exercise. I have been working on the exercise implementation, and I do not know why it was only implemented as an exercise-level interaction. I agree that having it as a task would make sense. I'll look into whether there is some technical issue that prevents that implementation.
Of particular concern for implementation, I am working on a redo of fill-in-the-blank that allows for the question to have random elements to the content and allow for the answer checker itself to account for this randomness. I do not think Runestone currently has a mechanism for separate tasks to share this type of dynamic content, so static tasks would be much easier than coupled, dynamically-generated tasks.
Brian Walton