Mutually Assured Destruction isn't the only possible game in town. There's also Nuclear Utilization Target Selection doctrine.
Obviously a mass ICBM launch aimed at completely destroying the enemy's nuclear deterrent, as an obvious precursor to destroying the enemy as a sovereign nation, would and should trigger a retaliatory strike of equal or greater magnitude. That's MAD, and that's the mutual deterrent that keeps the West from just ending Russia entirely, and vice versa.
But limited use of precision low-yield nukes to decisively end an enemy advance in a single airstrike should not trigger a mutally-destructive exchange of ICBMs. Russia would rather live to fight another day, then end the world just because its entire Ukrainian occupation force and supply train ended up in a radioactive crater.
And that scenario is where iodine pills would be extremely beneficial.