I think he was playing on words. Notice he wrote: flue, which is the vent that one opens in the fireplace when burning logs there.
You are probably right.
FB is probably working off TAF's GP post, which includes the interesting sentence, "It’s a total fallacy to blame the flue for all the deaths that they claim are caused by the flue." Note that TAF spells it 'flu' in the rest of his post.
Similarly, FB's post spells 'flu' as in the disease, except for the first sentence.
However, my bet would be that a higher percentage of deaths caused by faulty flues are correctly attributed than those attributed to the flu. E.g., does a 54-year-old die of the flu? Or something the 54-year-old would have died of relatively soon, anyway?
It is true that the symptoms of carbon monoxide poisoning resemble those of the flu. The main differences: The flu causes fever, CO does not. And CO causes death usually (or we wouldn't hear about it), flu usually not.