It's about knowing how to structure problems, whether the components are public or whether the problem space is completely unfamiliar. E.g. someone who guesses

It's about knowing how to structure problems, whether the components are public or whether the problem space is completely unfamiliar. E.g. someone who guesses "$3 billion" on only a gut feeling likely wouldn't be a strong hire even if the number is correct, while someone who can correctly decompose the final answer into its components and make reasonable estimates would be a much better candidate. For estimation interview questions, and problem-solving questions more generally, a "structural" approach to breaking down problems is important because on the job the interviewee will need to be able to identify what levers are available to them.