I overheard a pax at the Pre checkpoint at IAD arguing with the TSA, saying, "I'd rather opt out." I'm not sure what the heck he was talking about given this was the Pre checkpoint and has only WTMDs anyway. He went on at length about "opting out," but obviously got nowhere.

Maybe he was trying to opt out of the metal detector?

Normally people who opt out and get a patdown are not asked to walk through any type of scanner, including WTMD.

He may have had philosophical, religious, or other objections to WTMD as well as MMW.