Expert opinion is used for estimation in Agile?
Agile is just about values and principles. It does not say about how to do the estimation. Whatever method is used there should be a consensus for it. Estimation is the evaluation of effort necessary to complete a user story/task.
Yes, expert opinion is there, but it should not be confused with that it is dependent on the few experts. In Agile, participation based decision making is recommended, no one veto the decision.
Estimation in Story Points and Ideal days are the way of estimating, these are not rules. In Agile estimates are not same as commitments.
An estimate is not supposed to be correct; it reflects the information that was available at the time of estimation. When information evolves, estimates are also updated.