9.4.2 - Why C-style enums are exempted from requiring a default clause?

[cgpzs]

The problem with your example is that it already violates  10.2.3, so the question of whether it violates this rule as well is moot.

Fair enough. However, one could think of receiving said enum from an external C library, where we don't have control over how that enum is created. Wouldn't it still make sense to be defensive about those types of errors?