Getting good signal-to-noise ratio from ANY interview, forget the big cos, is really difficult. I've seen great programmers passed over because they were _not_ given a chance to whiteboard and show their technical skill. At the end of the day, there is a bit of randomness to the process of sizing anyone up in an hour, and people will make bad calls.
That said, I'm skeptical about your particular story. As a process rule, AWS doesn't provide interview feedback, so I'm pretty sure you don't _actually_ know why you were rejected, you can only guess. Perhaps if you were rejected in phone screen(s) it can be obvious, but if you made it to an on-site, I can guarantee (based on experience being in the room during hiring decisions at AWS) that they don't almost someone because of one bad whiteboarding result; there are multiple people collecting and reporting on results and only one person, the bar raiser, has veto power, and I've never seen it used for something as banal as not writing perfect code.
I also don't know what happened in your interview, but consider it's also possible that you completely bombed it: failed to ask questions, failed to communicate properly, or made more mistakes than you even realize.
It's certainly great that you went on to do great things, but maybe at the time you interviewed you weren't yet doing those great things...