I am referring to the meaning established in the law. Many things fall under the legal umbrella of sexual harassment. If you want to describe it as something else maybe you should be the one looking for a new term to coin.
No. The law does not describe a consensual relationship with a subordinate as "sexual harassment".
Obviously the situation isn't that simple, and neither is the law. If you really want to have a relationship with a subordinate there are legitimate ways to make that happen. He chose to circumvent those (ostensibly repeatedly, and while paying off people who found out about it), and the set of reasonable explanations that don't involve some form of harassment is vanishingly small.
Also, I don't think there was an "extreme power imbalance". If anything, she had far more power over him than the other way round, since he had much more to lose and she could blackmail him with making the affair public or other allegations.