The IDB laws were written to disincentivize the overbooking of flights for airlines and provide consequences for what were essentially intentional cases of IDB (the airline books it this way with the knowledge that people will be IDB). They don't have a law in the US (EU is different obviously) to provide compensation for things that are within airline control but unintentional, such as software glitches, mechanical delays, and GA idiocy, as long as the airline gets the pax to the destination. Most airlines have the practice, in those cases of "within airline control but unintentional", to make people whole for their hard costs such as hotel, food, and necessary purchases on the overnight but not for lost time, etc. It seems to me that this falls into that case where the airline should reimburse for hotel and meals, but asking for additional compensation is inconsistent with generally accepted practice. So yes, he was "denied boarding" and it was "involuntary," but it's not the situation that the DOT was trying to remedy when they made the regulation since it was an error.