It's obviously almost all confounders, since COVID mortality is low now and choosing to obtain the vaccine correlates strongly with other medical and non-medical factors affecting all-cause mortality. The point is that no adverse effect from the vaccine offsets that confounding beneficial effect. More covariates would shrink the confounding effect, but they did the best they could with their dataset.

That confounding effect turned out to be massive, which is bad news for anyone hoping to tightly bound the vaccine risk. It's good personal news for anyone in the vaccinated group, just more as to their general life choices than as to COVID.