There's still 5nm silicon for sale, but just not at TSCM (the largest semiconductor manufacturer in the world). Companies like Samsung are just now getting around to mass-producing 5nm, and afaik there were a few domestic Chinese manufacturers who claimed to be on the node too.

As for Amazon specifically though, I've got no idea. They're a large enough company that they could buy out an entire fab or foundry if they wanted, AWS makes more than enough money to cover the costs.