Amazon also has been running internally on containers for years before ECS. But workloads for massive FAANG companies designed by FAANG engineers turn out to be quite different than most AWS/GCP customers.
Just like EC2 wasn't Amazon selling its "spare capacity during off-peak" but always a purpose built service with completely isolated data centers and network fabric from day 1, the connection between Borg and Kubernetes is tenuous at best.
Honestly, I would encourage you to evaluate ECS and Fargate. Forget Kubernetes and direct instance management. Try a construct like https://docs.aws.amazon.com/cdk/api/latest/docs/@aws-cdk_aws... and see how much simpler it is than all the K8S overhead.
Source: I'm at Amazon but I work on none of these services and opinions are my own. I played with K8S and am eternally confused how it's as popular as it is. I guess it gives you the illusion of not having lock in?