CO poisoning can be related to partial pressure. The partial pressure is independant of atmospheric pressure or altitude, but as cabin pressure falls to say 8000 ft the effect of a set percentage of CO in the cabin will fall as the pp falls.
This is separate from the effects on oxygen delivery. CO displaces oxygen from haemaglobin. The partial pressure of oxygen in the blood falls with decompression so the effect of CO on oxygen delivery vs oxygen requirements will be magnified in respect of the tissues.
So yes you may get increased headache with altitude but conversely you should become symptom free more rapidly when back on the ground
Smokers will already have some CO on board from their cigarette so theoretically need less additional CO to produce symptoms
Of course all hypothetical as no CO demonstrated in any passenger nor any fault in the aircraft. But the Fire Chief had his moment of fame and possibly will be able to persuade the city to increase his budget next year to avoid thousands of future imaginary mass incidents.