It sounds to me like NullPrefix accepted the hypothetical bug, for the sake of the argument, and then addressed the rarity assumptions made about that bug. "For the sake of argument" doesn't mean "No debating" it means "Let's assume some thing x is taken as a given and go from there."
The whole point was that "given this assumed rare bug, one can know it's pretty rare". Whether the example is actually rare enough is completely beside the point. If it's not rare enough for you, replace it. The argument comes next: given a rare bug, a developer can make an assumption of how rare it is. Debating the example of rareness is splitting hairs.
