They probably mean that a large percentage of internet services are hosted on US-based servers. As a regular AWS/Azure customer, the regions in which I tend to see provisioning issues makes this seem right, though I'd love to see some numbers.
For various reasons the internet maps much closer to GDP than Population. But, largely it’s a case of smaller economies driving less advertising dollars which supports fewer websites. So it’s also weighted more to consumer spending than raw GDP.
Amazon cares a little about physical location and a lot about network bandwidth for this stuff.
So, they are building a location in South Africa that will cover southern Africa, but their locations in Brazil and Europe actually have good coverage for Africa already and only recently has the demand approached the point to be worth the investment.
The reason for the is under sea Cables provided most of Africa’s backhaul capacity, which relates a little or geography and a lot of economic development.