Your right though that candidates can get system design interviews, but "how many servers does it take to run gmail" is not anything like a system design interview. "How would you architect gmail" is. System design questions by their nature don't really have a correct answer. Your right though that candidates can get system design interviews.
Estimating how much traffic needs to be handled and what kind of resources it would take to handle it are absolutely part of both of those questions, which in reality are the same interview problem because a System Design interview isn't a single question but rather a long series of guided questions that cover architecture, scaling, and a lot more.
