AWS spawned off with Amazon realizing the value of dog-fooding, which is using their own products to build other products, and to sell all the steps. That is, the base premise was Amazon Retail being a client like any other of AWS. It is not a stretch to imagine the same goes for many AWS manages services to be built atop EC2. There are many products that are prime candidates for this: API gateway, Kubernetes, Lambda (which is likely built atop the managed kubernetes), etc

Heck, you can look at other cloud providers, and in some cases they are very open about stuff "being build atop public compute". One example that comes to mind is the VPC interconnect of GCP to get reachability between serverless and the rest of the VPC: it uses plain old compute to route the traffic; you can even pick which type and how many to provision.