k8s is not for everyone. It has a high administration and complexity burden. It would be a mistake to make k8s a requirement for core AWS services. Fargate is a low-level compute platform - it is parallel to ec2 and lambda. It does not compete with kubernetes (as can be seen by the eks-on-fargate offering).
There is probably some tension between ECS and k8s - AWS built a container orchestration platform based on what they think the world (and Amazon) needs and then k8s became madly popular. And it's not clear that AWS was wrong because k8s is essentially too complex for many use cases. It makes total sense that they would support both fully.