No, denied boarding means they won't let you board in the first place. In this case, he already sit in his seat.

While I suspect you're correct in this instance, that's not actually the point I was making. A lot of people have been going around the past few days saying, "I accepted a $20 voucher for my cancelled flight; was that illegal and they should have paid cash?!?!" People seem to have no notion of voluntary exchange.