People change their stories after a passage of time, because they didnt want to be involved (or get involved) at the time. It's a common syndrome. People generally avoid train wrecks or situations which could become overwhelming, and a tax on one's life. We have all been in situations and not come forward to avoid involvement at the time. Later, when people have distance, the full story comes out.
So, I wouldnt automatically dismiss what Cooper is saying today. It could be the truth.
In fact, I would bet others tried to avoid involvement for the same reason ... which alters the factual record. We have all been in situations like that...
He further advised that he did not observe the hijacker at any time, before, during, or after this flight ?