Notice that they are not using it as the only means for verifying system. They are using it as an additional check that supplements the extensive regression tes

Notice that they are not using it as the only means for verifying system. They are using it as an additional check that supplements the extensive regression test suite. The test suite failed in this case because the bug occurred only under high load (after this, they added a new test case that simulates high load). The human verification that was in place in production as an additional safety check operated as intended and detected the bug.