I can only find mentions of the minio client as a way of accessing their S3 service. Their introductory page[0] says "Any data you save in your Bucket is saved in a Ceph cluster.", which suggests they use Ceph's radosgw that offers the S3-compatible API[1].

[0] https://docs.hetzner.com/storage/object-storage/overview#obj... [1] https://docs.ceph.com/en/latest/radosgw/s3/