I’m also not convinced it’s a useful interview question because of the issue you raise, but thinking your mainstream views are heretical extends far beyond job interviews.
It seems like whenever I hear people talk about politics, they speak as though they’re persecuted lone wolves going against the current, but they tend to have opinions that something like half the country holds, which I would define as very mainstream.
I also think this occurs with myths that have been “busted,” someone will go around for years thinking that everyone else is sheep for still believing in the myth, when the reality is everyone else read the same article and no one really believes the myth anymore.
And then there are the countless posts on Reddit or Tumblr or Imgur or Facebook where someone dramatically makes a stand, admitting that they will be burned at the stake for their position, to the great applause of everyone watching.