Anecdata: Hetzner does not care what files are on their servers as long as you don't also serve them publicly from their servers. So if they only use it as a network storage for another server, it should be fine.
How is this abuse? You're paying hetzner for a VPS. If running a proxy server is an abuse of their TOS, then in my mind their service doesn't seem much more than a scam. I'm glad to know to avoid their service.