- Apple TV is bound to the device, not to Digitec.
- You can do whatever you want with your device. The first thing that is checked in a warranty claim is the moisture indicators in the Lightning connector. Apple rejects warranty claims for water damage: https://support.apple.com/de-ch/HT204104.
The waterproof to X metres is primarily an advertising promise. As long as a company doesn't give a guarantee, I wouldn't take it too seriously.
Just a possibility. A nice gift (or an attempt to win subscribers for the subscription). You can also just continue to use your Sunrise subscription and throw away the voucher.