The more typical definition of an extreme candidate is not "this person is not my top pick", but rather "I would not vote for this person at all". Candidates under RCV win when they get 50% of the vote, so if you're so extreme that only a small minority bothers to rank you then you're not going to find yourself elected on behalf of a minority. This is in contrast to FPTP, where any extremist can win with any miniscule percent of the vote if there's enough vote-splitting going on.