The number seems low, so I looked for some information about how common CO poisoning is in non-Airbnb hotels and motels and found this:
This study was conducted to determine the significance of the problem of carbon monoxide poisoning occurring in US hotels, motels and resorts. […] From January 1, 2005 to December 31, 2018, 905 guests were poisoned in 115 identified incidents, including 22 fatalities. […] Most poisonings were caused by natural gas fueled appliances and could likely have been prevented by an in-room carbon monoxide alarm. To reduce morbidity and mortality from unintentional CO poisoning in lodging facilities, government should mandate installation of in-room CO alarms, similar to the current requirement for smoke alarms.
This problem doesn’t appear to occur any higher in Airbnb establishments than any other hotel, motel, or resort, at least according to what this research suggests. It is worth noting, however, that the frequency of poisonings is a lot higher when you consider those that didn’t lead to fatalities.
With that said, CO monitors are fairly cheap and as the research states, a lot of these incidents could probably have been prevented by using them. In many countries in Europe at least, this is already a legal requirement. I don’t see why it shouldn’t be in the US too.