If someone can login to your Apple ID they can find your nsec if the app:
- stores the nsec in keychain and you have iCloud keychain enabled
- stores the nsec somewhere else on device unencrypted and you have iCloud back-up enabled
So to keep your nsec safe keep your Apple ID safe, or disable iCloud.
Also technically Apple can always get your nsec by pushing a malicious update to your device, but this is very tinfoil.