Thanks for the question. Smart Logbook does support creating backup files, which contain your entire logbook and can be stored anywhere you like, including other cloud storage providers (Google Drive, OneDrive, DropBox, etc). This is suitable for backup / recovery or device transfer scenarios. You can find more details about that option here:
The automatic, incremental sync functionality, however, only works with Smart Logbook’s custom built cloud service (and requires a sync subscription). The sync logic is non-trivial, as it needs to handle multi-device scenarios and extended offline use. It’s also highly optimized and very secure.