Can Airport Time Capsule encrypted backups be hacked open from internet?
Well first of all everything can be hacked. However you can make it as hard as possible. Use a strong password! Not abc123, password, your name, date of birth (easy to guess or find out on Facebook stuff) etc.

Time Machine uses the same encryption as FileVault 2, Kerberos, Keychain Services, Software Update Services etc. Encryption used is XTS-AES-128 encryption with a 256-bit key.

You can find more info on this page Product security certifications, validations, and guidance for macOS