Detailing what the aws doc means . It means that if in src bucket you specify a SSE-KMS ie you define the ARN of a key, than in the destination bucket you have to define a ARN of the key as well. Since Keys are regional and the S3 will not be able to make a DataKey API Call to that region if there is not key, hence it has to be defined. Hope it helps and you update the slide.
I haven’t tried this particular scenario myself, but I have used shared RDS snapshots using AWS KMS. In a nutshell, you can specify what KMS key you want to use (tip: you need to use one, other than the default), and once you have done that, you can share it with another account. Obviously, you will need to share that KMS key with the other account, and I would assume that works.