No, although it used (not sure if it still does) to encourage people to enable backups. On Android I believe the default was Google Drive, so you'd have people send their chats to Google in plain text.
iMessages is another example of a secure service that lets users "break" encryption. As soon we enable cloud features for it to work across devices, the key is uploaded to iCloud, essentially making chats plain text to Apple.
The main "backdoor" to Signal is that having access to the phone can leak all of Signal's data. If the phone OS is backdoored, then Signal is already compromised. Anyway, the point is not to make it impossible to exfiltrate data, but to make it as hard as possible.
iMessages is another example of a secure service that lets users "break" encryption. As soon we enable cloud features for it to work across devices, the key is uploaded to iCloud, essentially making chats plain text to Apple.