The conclusion for that study (almost 20 years old btw) stated the valence of the effect of helmets but not of the significance. The rest of the study has mixed results.
Here's a study from 2020 that further muddies the waters:
https://pubmed.ncbi.nlm.nih.gov/31335753/
"Helmet use doubled during the study period (43% to 81%, p < 0.001), but the rate of any head injury did not significantly change (49% to 43%, p = 0.499). On multivariable regression, helmeted patients were significantly more likely to suffer severe injury (odds ratio [OR], 2.01; 95% confidence interval [CI], 1.30-3.11), intracranial hemorrhage (OR, 1.81; 95% CI, 1.10-2.96), chest injury (OR, 1.66; 95% CI, 1.05-2.61), and/or lumbosacral spine injury (OR, 1.84; 95% CI, 1.04-3.25) than unhelmeted patients. Helmeted patients were half as likely to suffer cervical spine injury (OR, 0.51; 95% CI, 0.30-0.89) and a third as likely to sustain skull fracture and/or scalp laceration (OR, 0.30; 95% CI, 0.14-0.64). More patients who hit a stationary object were helmeted compared with those who fell from standing height onto snow (70% vs. 56% respectively, p < 0.001). After adjustment, hitting a stationary object was the injury mechanism most significantly associated with severe injury (OR, 2.80; 95% CI, 1.79-4.38).
Conclusion: Helmeted skiers and snowboarders evaluated at a Level I trauma center were more likely to suffer severe injury, including intracranial hemorrhage, as compared with unhelmeted participants. However, they were less likely to sustain skull fractures or cervical spine injuries. Helmeted patients were also more likely to hit a stationary object. Our findings reinforce the importance of safe skiing practices and trauma evaluation after high-impact injury, regardless of helmet use."
