If carbon monoxide poisoning from modern vehicles fitted with catalytic converters is not possible, how did 5?

When people say it isn't possible they are referring to the old suicide idea of running a hose from the exhaust in to the car. It's possible in the sense that eventually in a closed room enough would be produced, but with the catalytic convert functioning properly people aren't going to go through the effort it would take to get that much carbon monoxide.