Who has the skill to be able to assess such a system ? And among those, who has the time ?
I've been a programmer for 15 years and I'm pretty sure I would not be able to look at such a system and feel confident there is no error. And definitly not in the reasonable time period required for voting.
I believe the parent's example was to show that it was missed by the system that was intended to catch it. That the catch in itself was a fluke, lucky.
It should bring into question "How many times have we missed and not caught it?" not "Well it worked this time, it must always work." In fact, we can only recognize cases if we catch them. Therefore it is confirmation bias to use this example as a "proof of it working". It can only serve as an example of "We have caught failures in the system" and suggest that we should be wary of others existing (neither proving nor disproving fault in the system).