I don't think it's the server costs they care about it's stuff like db maintenance, security, security, and security. Security professionals are expensive, and it's very hard to get right in house on metal.
Moving away from on-prem data centers does not reduce security costs. At best, it's a lateral move, and depending on the cloud setup, may actually be more expensive.
Ultimately, this is an opex vs. capex decision. Data centers are expensive, and require a lot of up front capital to go into the ground. FedEx is worldwide and moving more technology to the edge, so buying land, designing and constructing buildings, and then operating them for 10 years or more in a large number of locales requires a huge investment. They can take that money now and solve their problems immediately.
They are also not saying what they mean by "closing its data centers". FedEx announced a 10-year partnership with Switch last year to build and operate edge facilities. FedEx may be moving out of data centers it operates on its own into facilities built and run by others, and using "cloud-native" designs deployed on hardware in those facilities.
