Yet they can still give off carbon monoxide so I'm rather puzzled.
Counter-intuitive as it may seem, a device that "burns" at so low a temperature that flame is not produced, probably stands a *greater* risk of releasing unsafe amounts of carbon monoxide, because carbon monoxide is one of the things left over from incomplete combustion.
I don't know much about gas, but with wood stoves, your risk of carbon monoxide is not when the fire is blazing at a good clip, but when it's burning low and the ignition of all possible combustables (such as carbon monoxide) is less efficient.
High heat burns off carbon monoxide.