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.
It first requires answer to the question "build what?" before the candidate can talk about "how". Unless they would build everything using the same tech, which also would be an odd signal. You putting a specific amount of money on it adds additional concerns regarding that.
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")?...
If the candidate is hopelessly stuck in a state of circular analysis paralysis then the quality and thoroughness of their answer would ultimately suffer. It would demonstrate either poor decision making, weak communications skills, or limited comprehensiveness.
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.
as an open-ended question you gain insight from the candidate's response to the scope with which they think about things (and whether that matches up with their experience)... also 100K isn't much so it should force some critical/creative decisions
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.