For starters, as a baseline, you need to be very sharp at coding interview style questions. This isn't fizzbuzz. Based on my tech interviews at google and a few other companies, I'd say a question like [1] "find the sub matrix with the largest sum in an NXM matrix" is something they really will expect you do complete at a high level of accuracy in 45 minutes at a whiteboard, including run time efficiency improvements. You'll get some help, and they won't be complete sticklers about syntax, but they will largely expect you to writing accurate code that would compile and run properly without too much help. You'll need to be able to follow through on questions like this in a areas, such as binary, algorithms and data structures, and some operating systems issues (threads and processes, in particular), along with good overall design skills. While it is absolutely possible to get to this point without a CS degree, and many people do, it requires an academic talent, mindset, and ability to focus.
That's the baseline. Generally, that's the ante that gets you in the game. I think there's a way around, which is to create your own projects that are so high profile that you get hired directly into a company at a high enough level that you bypass the entry exams. However, this is rare, and plenty of people who have excellent track records are still filtered out at the interview level.
After that, you need savvy. My observation after 15 years in the field is that the people who are considered technically exceptional are also organizationally savvy. They're not fakers, at all, they really are good technically, but they understand how to pursue the good projects and avoid the bad ones. It becomes self-affirming after a while, their reputation allows them this kind of selectivity. But they don't get stuck on legacy projects that don't enhance their skills. They are able to navigate organizations well, and this requires political and social savvy.
In short, these people are both very strong technically and have the social savvy to get high profile projects where they will continue to learn and gain recognition for their achievements. This combination is relatively rare in humans.
I will agree that this kind of person has opportunities in the valley that are available only to a lesser degree elsewhere, and may not be available at all in some places.
[1] NOT an actual question from my interviews, but at about the level I'd say I experienced (maybe even a bit easier). I highly recommend the book "cracking the coding interview", just be aware that yes, really, they will expect this sort of thing in 45 minutes at a whiteboard with accuracy and efficiency.