It's less true for Omicron, but the study there looked at transmission in Danish households. Against Delta, vaccinated individuals were much less likely to get infected. Against Omicron, boosted individuals are less likely to get infected, and vaccinated are similar to unvaccinated.
"This article is a preprint and has not been peer-reviewed [what does this mean?]. It reports new medical research that has yet to be evaluated and so should not be used to guide clinical practice."
Consider all pre-prints complete bunk until proven otherwise. Anyone can publish whatever they want to support whatever conclusion they want in a pre-print.
It's an analysis of an interesting data set and the conclusion isn't unreasonable.