" "I'm surprised that physicians would prescribe melatonin for insomnia and have patients use it for more than 365 days, since melatonin, at least in the U.S., is not indicated for the treatment of insomnia." "

OK, I'll bite; why the heIlelse would a doctor prescribe (or an American buy, OTC) melatonin if not for insomnia (or insomnia-like) conditions?

That aside aside, the article isn't bring the perennial issue into sharp focus, which is poor medical journalism: causation or association? One can easily posit that insomnia is a prodromal artifact of some underlying, and subclinical heart dysfunction. We can't determine from the data how many of these heart failure individuals would have died earlier had they not taken any melatonin at all.
Or more simply, had they quit melatonin, would they have died earlier (because insomnia started wrecking their health in earnest), or would they have died later (because melatonin was killing their heart)? We don't know. The article isn't telling me to stop taking melatonin.