I think you got two things confused here. First you talk about addons needed to get a world 1st, then about needing them to kill a boss. Those are two different things - he never said you are unable to kill a boss if you don't use said addons to the max of your capabilities - he said you won't get world 1st without it. Which is rather obvious, since in a race of the very best players in the world, you really need to use every small thing to your advantage.
But that hardly means good players are unable to kill bosses without addons. Pretty sure they can, it will only take more practice and patience to learn the ins & outs of every encounter (and maybe a bit higher ilvl to offset the fact that you won't be able to use every cooldown as effectively). But why do that, if the addons exist? Only to prove some point?