That's not a very useful study because of this:

Third, because previous infection could not be measured, its effect on VE estimates could only be inferred, not quantified

Due to the staggering numbering of Omicron infections over the last year there are very few unvaccinated people with no immunity. Using them as a baseline is going to give a very inaccurate estimate of protection.

For a better picture look at Singapore. They have very high rates of vaccination but haven't rolled out the Bivalent booster yet. Look at and their recent XBB wave and compare that to Hong Kong's Omicron wave earlier this year an you'll see clear as day that vaccines continue to offer strong protection against serious illness.