Yes, it is idempotent. It means, when the target file on the local drive matches the source file in the S3 bucket, Chef Client does not download the file. In the past aws_s3_file was not idempotent, but now behind the scenes it calls remote_file which compares the files and skips the download when the files match.
* aws_s3_file[/var/myfile.deb] action create
* remote_file[/var/myfile.deb] action create (skipped due to not_if)