The LD50 for Zopiclone is very high when taken alone. The source you cite says most people who died mixed the drug with something else, but the problem is that there's usually very little data on the lethality of drug mixtures. For example, if you do the classical "Benzos + Opioids" OD, yes sure those two drugs potentiating eachother likely lowers the lethal dose substantially (relative to what it would be for taking one drug individually), but I wouldn't have any idea by how much. And then it's just a shot in the dark how much to take.
Talking about your link itself, it's fascinating that the highest (plurality) proportion of OD's came from mixing with AntHistamines. That's a really crazy combo that a bunch of benadryl and Zopiclone would take you out. But people mixed with all sorts of stuff. And the study only includes the fatalities, it doesn't seem to speak to how many people have actually attempted using this OD method (which would be necessary to figure out the fatality rate).
That being said it does give a median blood concentration of people who did succeed in killing themselves with the drug. BUT these aren't monointoxications, so we don't know what the concentrations of other drugs were when they dosed on the Zopiclone.
The hunt for the magic OD continues...