They’re not saying it is flat out deadlier, just that the typical user is more likely to overdose on alcohol rather than heroin. They look at the lethal dosage, versus the typical amount consumed, and they found the ratio is higher for alcohol. That doesn’t seem far-fetched to me a all, since people drink to get drunk, and it doesn’t take much to get from drunk to organ failure.
There are 20 million alcohol abusers and 1/2 million heroin addicts in this country or a ratio of 40:1. The ratio for total users is higher of course since casual drinking will lead to addiction much more rarely than heroin use. There are 2 heroin deaths per 100k total population annually and 28 per 100k for alcohol. Heroin is thus proportionally more deadly.