Your interpretation of the subtext in the OP is predicated on context which does not appear in the text. Answering a question with a metaphor implies that the metaphor will demonstrate an answer to the question; nothing more. It does absolutely nothing AT ALL to suggest it is an invalid question; you've just made that up. The respondent being analysed has in fact recognised this subtext equally and their resulting lack of understanding has happened in spite of this.
In my math class example although the test question was a written question, I received it in person in math class in middle school in rural Australia during late 90s from a teacher and as a part of a syllabus I was familiar with. These are just some examples of the contextual clues which in combination with the text formed the subtext or the basis for my interpretation of it. There are other circumstances I've not mentioned because they are irrelevant to the point I was using the example to make, and it's none of anyone's business. That said, it is just plain ridiculous to argue about the subtext of a question paraphrased in recollection after decades as if my original comment has somehow given you a more accurate read on the experience I lived.