Having an unrecoverable stall mode is not a desirable flight characteristic, for sure, but I don't think it's unheard of. For sure it's nothing like inherently unstable airframes as used in fighter jets.
My rather limited understanding is that the airframe certification path Boeing used was one meant for fast-tracked recertification of minor configuration changes of the same hull. The main idea being that the manufacturer need only prove that the reconfiguration meets the same or similar flight characteristic requirements as the hull it was derived from.
This lets them produce/sell a technologically refreshed product without having to pass the rigorous testing normally associated with type-certifying a new airframe. The important characteristic of this is that the onus for this certification is left to the manufacturer. I do not know if the FAA actually independently confirms the recert is justified, or just plays it via honor system.
Old 737 hulls definitely didn't have this characteristic onstability, or do-or-die reliance on functioning AoA sensors, so while I agree there isn't anything wrong with incorporating instability into a design to satisfy some specifications per se, I'd still contend that the addition of that instability should disqualify an airframe from being recertified as a reconfiguration of an older one, and should require more extensive testing/training before pilot's are cut loose on it.
If anyone else has more accurate understanding of type certification, I'd be thrilled to learn the specifics.