ironically, adcs are stronger against assassins overall when they aren't strong enough to be priorities in fights.
While this does sound kindof backwards keep in mind that both teams have access to this damage source.
scenario A
ADC is weak but still good at taking towers down
teamfight starts and the assassin kills the enemy adc, the allied adc then procedes to do nothing special and the enemy team attacks the assassin who tries to escape and they maybe cant really be stopped
scenario B
ADC is strong as shit
teamfight starts and the assassin kills the enemy adc, the allied adc is thus free to mow down the rest of their team and they have to retreat or manage to kill the obvious prime target.