I think the point is that if your data is valuable enough for you, you can't really trust that option in the backup tool to work - maybe you misunderstood some config option and the test now isn't really run, the tool is broken, or is run only on some of the backup files or dirs etc... or your original config might have missed a folder because it was mounted through some other filesystem (happened to me with Borg actually, and my whole /home/user dir especially wasn't backed up for the first 6 months I ran it :).
Seems to be good to have another tool that you either manually or automatically can setup to run regularly that tries to locate random files from your existing file system in the backups? Something like that.. though that other tool might be broken as well of course... :/
It's a very hard problem. In the end everyone is having an increasing amount of data where double checking it manually is not feasible any more and a perfect solution is maybe not possible.
Reling on software with good defaults that a lot of people use is probably a relatively safe bet combined with a second or third backup system (Personally I use Backblaze and Time Machine).
Seems to be good to have another tool that you either manually or automatically can setup to run regularly that tries to locate random files from your existing file system in the backups? Something like that.. though that other tool might be broken as well of course... :/