Backups should always be tested atleast once to ensure that they do in fact work. Nothing worse than losing data, and discovering that you've been paying a service for naught all those years. This could get prohibitively expensive if you can't replicate your setup in the cloud to test backups within AWS network.
Or you could just blindly trust that it'll work. Kind of like schrödingers backup
Could be, but it could also be a fluke. Perhaps everything after the first file-index file is corrupted, and you only restored things from the first one, or perhaps maybe the entire diff-index can be so large that it's not actually recoverable once you grow it to the size you want.
There are a myriad of easy-to-miss failure scenarios that don't appear in small-scale tests. Only way to ensure you can restore your backups, is to restore your backups
Or you could just blindly trust that it'll work. Kind of like schrödingers backup