Rammus is still able to..

this ^

{quoted}

Let's say the ADC has 1,500 health only for scenario sake since literally all of them will have more at level 18. Let's also assume that they are not building any MR whatsoever and therefore only have the bare minimum magic resist of 30.

A rammus with 300 armor would get to 590 armor -- I'll increase it to 600 for giggles. The ADC would need to attack 23 times with no lifesteal in order to kill themselves. This means that in 2.5 seconds they would need to attack over 9 times a second...

so we disregarding rammus ulti autoattacks etc. Only the adc hitting and rammus standing still? no shit it would take tons of autoattacks to die simply to his thromnail+W.