I don't think Apple have any interest in strictly enforcing this rule. It's meant to be a catchall clause. Most likely App store reviewers will first use some subjective criteria like whether an app feels like a native app or whether it fits certain performance requirements. If an app fails to match the subjective criteria it will be submitted for further inspection. And just then, if it fails to pass that one too, it will be "objectively" rejected for a subjective reason.