As I read it, the issue he's talking about is that it's easy to accidentally write a test that passes the first 100 000 times you write it, but then fails the next time because of a timeout that was set too low or something like that. A test like that can waste a lot of your time tracking down a nonexistent bug.

It's true that any particular test that spuriously fails one in a million times may never fail. But if you have tens of thousands of tests, and you do tens of test runs per day, you'll have a test spuriously fail once a day or so.