My opinion on this issue is that there are many ways for people to get through interviews ahead of other qualified people, it normally comes down to being better prepared for the interview, or being a better "fit" for the organization, or just whether you present well in the situation.
The same thing applies to tests, exams, "challenges", homework, etc.
If a candidate is willing to take the initiative to prepare harder for the interview, isn't that in fact a good thing?
The information about what questions are asked at a company is going to be in many places, not just Code Review, and I am delighted to know that people using Code Review as a reference may just have better answers than other folk... good for them!
That is "obviously" ignoring people using Code Review or other sites to help them DURING the test, or interview, but anything that gives people more confidence, skill, or references for them to use in an interview can only be a good thing.
As a person performing an interview, I expect people to be able to reference google for answers in their day-to-day life, the interview process for me is more an attempt to get to know their "baseline" before they have to reference the search engines.
So, to your question "Are responses to specific interview questions on-topic?" Yes. Yes they are, as long as they follow the normal answer requirements (they are actually code reviews).
To the people who come along afterwards, and find your answers, and it helps them get a job, well, good for them for researching before the interview.
The real question is whether it is OK to post the QUESTION, not the answer... but that's a different topic altogether (and the only person who can complain is the "owner" of the question itself, and if they are concerned, they can request that their copyright material is removed).