Unfortunately, this is a myth.
In the commonly-imagined “spoiler” scenario where there are two strong candidates and a few fringe candidates, RCV does allow you to vote honestly for your favorite fringe candidate, who will be eliminated and your vote transfer to the lesser of two evils.
However, if there are three or more strong candidates, RCV still has a spoiler effect, and voting honestly for your favorite could help the greater of two evils win. So no, it’s not safe to express your first choice under RCV.
You need a voting system like Approval or Score to make it truly safe to honestly vote your first choice.