As such you are right: you can retrieve your files from a sync store to DEVONthink (using DEVONthink & to go) - so if you set up a new device, you can sync all your files to it from a sync store. That is assuming all your files have been put there, and the store is intact. But: if you suffer data loss on a device (accidental deletion, malware, corruption), that loss will be propagated to your sync store and any attached devices. There is no versioning - so sync fails even the most basic test for a backup.
Sync is great - it is not backup. If you are interested in backup, some users - myself included - posted their ideas on strategies here.