This study bases its conclusions on a number of case controlled studies.
Goldacre and Speigelhalter discuss the limitations of case controlled studies in their BMJ editorial.
This finding of “no benefit” is superficially hard to reconcile with case-control studies, many of which have shown that people wearing helmets are less likely to have a head injury. Such findings suggest that, for individuals, helmets confer a benefit. These studies, however, are vulnerable to many methodological shortcomings. If the controls are cyclists presenting with other injuries in the emergency department, then analyses are conditional on having an accident and therefore assume that wearing a helmet does not change the overall accident risk. There are also confounding variables that are generally unmeasured and perhaps even unmeasurable. People who choose to wear bicycle helmets will probably be different from those who ride without a helmet: they may be more cautious, for example, and so less likely to have a serious head injury, regardless of their helmets.
A quick reading of your link seems to show that these mistakes are repeated.
There is no acknowledgement that whole population studies fail to show any benefits of wearing.
With regard to your previous question, it seems that cyclists forced to wear a helmet by law will tend to be the risk takers in the cycling population, so helmet efficacy would be overstated in compulsion countries, when before law and after casualty rates are compared. That is, if helmets are efficacious.