Yeah, if the exercise is in using a poorly documented API, and the answer depends on fiddly bits of trivia around how the response is structured, then a debugge

Yeah, if the exercise is in using a poorly documented API, and the answer depends on fiddly bits of trivia around how the response is structured, then a debugger is absolutely the right approach, even during an interview. You did the right thing here, and the interviewer is a dummy.