The question is rhetorical, so what does this matter? It is part of a job interview. It is not a VC pitch. If this confuses a candidate and they answer a question not asked I would not hire them. Interpreting and following instructions are important skills.
You are select for the "can bullshit people on short notice" ability. It is very likely not correlated with other abilities required for the job, thus, if it is not required, you will very likely fail the stronger candidates because of that.
Where is the complexity in the question and how is it poorly communicated?
Do you expect the candidate to talk about some tech stack and/or development process to you? Do you expect the candidate to describe what they'd work on (then why don't you ask that?)? Do you expect the candidate to to treat the question as "if you could work on whatever you liked", or is it "what kind of product would you build", and then talk about the process? Do you expect the candidate to ask for clarification, or is that a bad sign already (since they were to dumb to understand your "simple question")?...
There are a lot of candidates who need to have their hands held by a parent authority or are hopelessly incapable of forming original ideas. This is a means of identifying such people so that they can be removed from consideration.
> Do you expect the candidate to ask for clarification, or is that a bad sign already (since they were to dumb to understand your "simple question")?
If I had to answer the question for the candidate, then yes, I would consider that a bad sign. It is a simple hypothetical question demanding a hypothetical response.