The best technical interviews I've been on as a interviewee have been those where the expectations are clear. In your example:
"We're not expecting you to create Twitter in 15 minutes, but we want to understand how you think about the challenges and key considerations of building a large system like Twitter"
Many interviewers fail to provide enough context and that leaves the interpretation of the prompt too wide open. At that point, the interview has failed since whether a candidate can provide an answer that is aligned with the expectations of the interviewer has an element of chance to it.