Perhaps some omnicidal maniac with a lab could engineer a weaponized virus that could be as dangerous as a nuclear war but I consider that a more-or-less fanciful scenario. A nuclear war is not a fanciful scenario.
I think you're missing both the Law of Large numbers and deflationary trends in biotechnology.
If people are thinking existential risks, and some of this thread is that, then people are missing something if they're ignoring bioweapons.
I think Putin's risk is nuclear extortion being tolerated, and its ongoing influence on realpolitik, than actual nuclear annihilation