Useful aphorism in general, not terribly applicable in this case. Even if it had been tested regularly, it was cut around the same time the primary failed. Regular testing doesn't stop construction crews from cutting a line.

A more useful comment might have been to say something about the depth of the backup, having only one backup connection not being enough.

If you are curious: one of the most frequent problems that disables fire sprinklers is maintenance forgetting to turn the water back on. No one notices until it is too late.

No one notices the thousands of times the redundant links works just fine. We only notice when the primary has an outage and a construction crew cuts a fiber line at the same time.

If you’re relying on your redundant link thousands of times you probably need a third link.