Google didn't reject the candidate, the interview loop he got rejected him for any of the myriad of knee-jerk split-second decisions made on the candidate's character in an unnatural and stressful environment.
I've been in hundreds of interview loops at $Megacorp. You can stack the deck with preparation, but sometimes you just hit a loop that doesn't click with you.
As a candidate, if I do not know, I say I do not know. I can offer my thoughts on "but if I were to design this from scratch" based on my experience, but only if the interviewer is interested in that.
As an interviewer, I hate when candidates pretend (or believe) that they know something when they actually don't, and just confidently make stuff up like ChatGPT.
But that's not how problem solving works. You need work your way from what you do know to get to an answer for what you don't. It's not guessing -- it's taking the background knowledge you do have and applying it to the problem at hand (and every engineer should have some background knowledge, even if it's just basic arithmetic).
That's what these kinds of questions are testing. Someone who immediately gives up is probably not going to be resilient in the face of new challenges.
I've quit several interviews because of these particular questions since they are simply wasting my time and not testing my "skills".
Well that's the opposite of what the interviewers are looking for. They DO want to work with such people.