I think they're saying that their understanding of the hypothetical error is that somebody more or less configured a price as 5¢/byte instead of 5¢/GB. And if that's the case, that's not really a thing testing will catch. 5¢/byte is as "valid" a price as 5¢/GB.

There's a lot of hypotheticals in there, so who knows if testing could have caught this specific problem. But I see what they're saying in principle.