Not being able to back up app data to another device that I own because its encrypted with a hardware-tied key that I can't access is a terrible design that plagues modern mobile devices. It's my device; I should be able to access any data on it.
There are plenty of viable security models where the app data could be securely protected from Apple or Google by using a key that I own and can use on other devices.
That sort of feature is dangerous, and very likely to cause at least one vulnerability. If I were running signal, I would be very reluctant to, and careful in implementing it.
Anytime you use a device you need to trust the manufacturer itself is not malicious. The difference between storing and decrypting a database on an internet-connected Apple device and storing it in Apple’s iCloud is minuscule.
Then it doesn't respect the users enough to be considered free software. The user must be free to do whatever he wants, even sending a plaintext copy to Google.
There are plenty of viable security models where the app data could be securely protected from Apple or Google by using a key that I own and can use on other devices.