No, Microsoft Defender for servers does not require Azure Arc to extend protection to hybrid cloud workloads, including servers running on AWS.
Azure Arc is a separate Azure service that enables you to manage servers, Kubernetes clusters, and applications on-premises, at the edge, and in multi-cloud environments from a single control plane. It provides a centralized management experience and enables you to apply policies, update servers, and deploy applications across your hybrid cloud environment.
However, if you want to use Azure Arc to manage your servers running on AWS, you can do so by using the Azure Arc enabled servers feature. This feature allows you to onboard your AWS instances to Azure Arc and manage them through the Azure portal or Azure APIs. In this case, you can also use Microsoft Defender for servers to extend protection to those AWS instances.