In an EC2 you can install OneAgent, and that will consume HUs. If you monitor it with AWS integration, you'll consume DDUs, plus AWS API usage. The latter option has a much lower observability than having OneAgent installed.
With OneAgent installed on a Operating System you basically get full-stack visibility into the OS metrics, the processes that are running on the machine and deep monitoring for processes, their provided services and applications.
Basically this is the key feature of Dynatrace. The AWS Api monitoring is "just" fetching metrics provided by AWS and puts them into Dynatrace and connects them with the full-stack monitoring.
If I'd put numbers to it I'd say OneAgent full-stack monitoring provide 95% of the value and the AWS api the remaining 5%.