You're kind of proving (part of) my point?

How? Their reliability would exist without that. There's nothing inherent to their own data center that makes their setup that much better. Having a distributed system across multiple cloud service providers means your actual chance of downtime (here I mean inverse of uptime) is their individual chances of uptime multiplied by each other. In other words, they all have to go down for your service to fail. The catch is you have to use only commodity IaaS and PaaS, nothing proprietary to one CSP.

For smaller companies especially, in terms of pure reliability, there's no reason to think that they would be better at running a high availability data center than Microsoft or AWS or Google.

Parallel distributed architectures give you the advantages of using public cloud (not having to physically manage your own data center) without the disadvantages (dependence on any one cloud vendor), while also potentially increasing your reliability beyond the reliability of any one of your cloud vendors . That is why Netflix is so rock solid.