The problem was a mistake in the arguments of the service declaration (which I should've seen). It needs to be a collection like so:
config.yml
services:
src.aws_s3.client:
class: Aws\S3\S3Client
factory_class: Aws\S3\S3Client
factory_method: 'factory'
arguments:
- key: %amazon_aws_key%
secret: %amazon_aws_secret_key%