tldr: Omicron likely doesn’t protect the unvaccinated against other variants.
opinion: So Delta remains a significant health risk for them, esp. in light of lingering post-recovery issues.
These are the same types of studies which were used to estimate that those with natural immunity would do much worse than vaccinated during the delta wave. Months later according to CDC those with prior infection were many times more protected from negative clinical outcomes. Why would someone use this low statistics serology study to infer clinical outcome when in the past they have got it wrong?