This is partially true with respect to EBS (i.e. virtual disks). When you boot up an EC2 instance, the a plaintext data encryption key is loaded into hypervisor memory. As long as the EC2 instance is still running, you can add an additional EBS volume to the instance and then copy data from the disk that has the orphaned key. Even if the underlying KMS key was deleted.
The rest of this stuff feels like FUD me. Yes, IAM has identity-based policies and KMS has resource-based policies. And yes, default service KMS keys are unique per account (why would you expect otherwise?).
Implementing an encryption program takes forethought. This is true if you're hosting your own data, too. I really don't miss manually rotating drives, fighting with LUKS, and then putting drives in a fireproof safe (which was never locked anyway, so I'm not sure if it would've actually protected anything in an actual fire).