In my experience failing to answer the alternative question you give me has (on average) a much more negative impact than pretending I don’t know your question (especially when I can explain it).
As an interviewer though, there were a few instances where I just told people "let's do this problem anyway, don't worry" and the candidates didn't always do a good job.
This is what I've done as an interviewer. But then, my questions actually lend themselves to that. Something that the sample LeetCode questions just don't.
In my experience (as interviewer) LeetCode kind of questions are "gotcha" kind of tests: Either you know "the trick" or you don't, but there's no real constructive value.
In contrast, I prefer tests like let's write a Tic-Tac-Toe, Snake, Twitter-clone (no DB in memory), etc. in 30/60 minutes together in your computer with your IDE, your software, google and language of choice. I am able to do a quick coding session with the person, see her weaknesses and strengths, and even if he has done it before, looking at his real project coding style is super useful.
Which should be a clear indicator that interviews aren't only judging problem solving skills, but also the candidate's ability to withstand the pressure of being watched and judged while they solve complicated problems.
For the interviewer, it's just another day and sure, they "want the candidate to succeed" and all that, but for the candidate, their future and livelihood are on the line and that's tough for some of us to just ignore while we focus on the not-easy school quiz problem of merging overlapping intervals or whatever.
In a perfect world every interviewer would calibrate for those minutes lost, but that calibration is fuzzy at best unless it’s built in to your scoring rubric.