The CO component of the story has zero credibility:
For one, CO is the product of incomplete combustion of carbon compounds under oxygen deprived conditions AND high temperatures. The only place on the aircraft where it may be produced in measurable quantities (barring an on-board fire, which was clearly not the case) are the combustion chambers of the engines and the APU. As stated several times above, if the engines are producing thrust and there is sufficient airspeed to maintain flight, it is physically impossible for any CO produced there to enter the cabin air supply.
The fire chief was more than likely influenced by cognitive bias. They would have received extensive training on the effects of CO poisoning, as in case of victims of building fires swift recognition and treatment is vital. Naturally he would 'recognise' any condition with similar symptoms as CO poisoning, without considering other options. Very similar to doctors with different specialisations tending to come up with varying diagnoses matching their fields of expertise for the same patient.