"How do you make sure that a celebrity's tweet reaches all of her followers in less than 3 seconds?"
Looks like the interviewer has atleast read the first chapter of "Designing Data-Intensive Applications: The Big Ideas Behind Reliable, Scalable, and Maintainable Systems"
If you read and understand the above book, I am reasonably sure that you can crack most of the system design interviews.
Even if you are not working on a product that requires supporting millions or billions of reads / writes, knowing what is overkill and what isn't, for your project's use case is still useful
System design interviews IME are very applicable/productive actually. Especially ones that put you in a non-greenfield scenario.
Getting good at those requires studying topics that actually are relevant to the day to day job. The Kleppmann book referenced above is one of the bibles IMO. A software book that's actually worth the paper it's printed on.
I could be missing something about US software culture - is this considered a must-read or "celebrity" book over there?
But the % of devs in the US that have read it is probably below 2-3%. Most of us (devs, not HN users) don't read or do code after work.