It looks like AWSS3.s3_list_keys returns a Base.Generator. This is kind-of like an array, but the elements are only evaluated individually, usually to be processed one-at-a-time, which avoids array allocation. See Multi-dimensional Arrays · The Julia Language or What is a Base.Generator?.
Try collect(AWSS3.s3_list_keys(CephConfig, bucket_name)) to evaluate the items and store them in an array.