Replace what “S3” with “S3-compatible object storage”.

AWS is almost never the answer unless you work at making it cheap and work for you. It's a black hole of insanely convoluted billing and filled with snake oil salesmen (dev ops priests) that'll complicate it so much that you don't even know what you're paying for or why you even need it. And S3 is just a gateway drug into this whole mess, stay far away from it kids.

I think he was being sarcastic

I'm talking about storage volumes which are attached to the VM versus object storage which provides S3-compatible API.

You can use S3 API to access Backblaze.

I'm not experienced with Hetzner Storage Box but I don't think that you can just attach it to your storage VM as fast storage. You can mount it as samba store but I think that's a recipe for disaster.