It's not about the burst. It's about the fact that it's Burst that happens during an Unstoppable Status that is followed by a knockup long enough to kill you before you can even skill.
Most AP champions can be mitigated, because the burst doesn't all occur at one time, and because there is time to react/counter CC
Furthermore, his scaling was outpacing both Magic Resist builds AND HP builds, so when most champions building against him would have ~ 2000 HP, he'd still be doing 2100 post mitigation.
He was routinely doing this every game, and the ONLY reason he is 50% is because his leads were novelties; He would kill 1-2 people, and then be useless for a minute +, and often would be cleaned up by the allies of the champs he killed. Teams would take advantage of the gaps. Nothing more. Nothing less.