> Instead, the routine likely should have worked within `Result`.

But it's a fatal error. It doesn't matter whether it's implicit or explicit, the result is the same.

Maybe you're saying "it's better to be explicit", as a broad generalization I don't disagree with that.

But that has nothing to do with the actual bug here, which was that the invariant failed. How they choose to implement checking and failing the invariant in the semantics of the chosen language is irrelevant.