That’s not what the upgrade does. All the upgrades for Assassins and Abductors do is prevent them from applying the Jailer’s Mark debuff to you which deals damage over time and ticks up your Eye of the Jailer level every time it deals damage.
It’s still quite a useful upgrade since the Mark’s damage isn’t exactly trivial and it additionally buys you extra time in the Maw for each assassin you spawned and didn’t have the Mark applied.