Literally all of them. Availability might have been a good point, but their other blog post somehow reaches 100% so I don't believe that either. Also not every business needs 99.9999 availability and can give up for good reasons (like cost and complexity).
The author is an engineer at AWS so he has an obvious bridge to sell.
That said, I do agree with the general point of needing more than one computer. But most people probably just need two, one for backup, far from “distributed system” in the traditional sense.