In the context of their business -- ingesting data that will never been read -- and architecture[1], low average transfer per disk is to be expected. Prior to the v5 Pods[2] they were on 1GbE networking as well.
[1] https://www.backblaze.com/blog/vault-cloud-storage-architect...
[2] https://www.backblaze.com/blog/cloud-storage-hardware/