When we look at the entirety of companies around the world, it's reasonable to think that there are non-FAANG companies that could be just as skilled. That stated, my own casual observations:
1) The FAANGs that I've seen have significantly more robust practices than the non-FAANGs. Everything from office conditions to build / test / release systems. The reason for this is:
2) Incentives. When your business is completely focused around software technology, you are incentivized to build out systems and processes that other places would not, because it is core to your business.
Anecdata from many years in and out of various kinds of companies, as both an employee and/or road-warrior consultant.
Well at least they are trying to be. Some of them employ PhD's in theorem proving to apply computer assisted proof to improve their software (and hardware).
Who's doing this? Amazon is using model checking on simplified models of real-world software systems, that's not "theorem proving". Microsoft is doing some related stuff in low-level code (Sing#, etc.) but they're not in FAANG. Galois is not even close, they're a relatively small shop.
To accompany the neighboring reply from the grandparent poster, the practice of site/service reliability as an engineering discipline is not just popular amongst the FAANGs, all of whom have SREs, but embedded into the company culture and structure. Google, for example, has a head honcho of SRE (a VP of SRE?) who gives all SREs extra authority to act somewhat independently.
A large part of the perceived competency of the FAANGs is their ability to casually and repeatedly achieve 5+ nines of availability. When HN is unavailable, that's no big deal; when google.com is unavailable, users act as if an apocalypse has begun. That difference in reliability changes a service from something that people consider as an option into something that people rely upon.
