Yes, Google backs up to tape. I've seen it done and helped design the training for the process. As of 2019 it was a robust backup process with a pain in the butt restore process. It can and will be done, but it's not fast or easy.
When I was there Google used multiple Sun/StorageTek tape libraries at each datacenter. The robots handled much of it, but there was still a lot of manual tape loading and unloading performed by technicians. Once tapes were full they were unloaded and hauled off to a secure third-party facility.
