According to the article, there was a "gap" between the exhaust and the engine. So the hood vents were not working, clearly. Might not have secured the header properly, allowing a small gap that could allow CO2 and not much smell to escape. Had they been drinking a little, they would have mistook the initial effects of CO2 poisoning for an alcohol buzz, and by then it is too late. Modern cars, especially in the UK, produce so much less CO2 then the boats of the 70's, but they are also sealed up much better.