ExXB, it's clear that the law assumes that commercial decisions which leave passengers behind on the ground constitutes come form of misconduct, which must be punished through the DBC scheme (and make no mistake - the second DBC package was intended to be punitive, not just compensatory).
What the law overlooks, as in the example you quote, is that a commercial decision to deny boarding may be actually be made so as to inconvenience the smallest number of customers. If you provide a 124-seat aircraft for 180 passengers then you are 46 overbooked and you must bear the full weight of DBC which at its core is an anti-overbooking instrument.
If you provide no serviceable aircraft at all for 180 passengers, then in the spirit of the legislation at least, you haven't overbooked them.