I read the parent comment as questioning the wisdom of relying on a cloud provider (but not specifically AWS, as you indicated) for a global operation life FedEx. Even with service zoning, in extreme cases that provider constitutes a potential single point of failure. Unless FedEx are able to run their core systems on Azure and Oracle‡ then I think the point stands.

It would be interesting to know how Fedex sees the risk/reward equation, and how Azure (for example) compares to a (IBM?) mainframe data centre in terms of reliability and uptime

‡ I'm not sure if there are any cloud neutrality solutions that permit this, but it is possible that FedEx have rolled their own.

Hard to say for certain. Companies that size often have many very independent operations run by separate internal departments. I have worked places with internal, AWS and Azure operations underway.