Barring some sort of destructive external catalyst (on-ground impact, midair collision) uncontained engine failures were supposedly relegated to the "nearly impossible" design scope.
Where has this been claimed? This exact same thing happened to Southwest a few years ago, just without the loss of life. https://en.wikipedia...nes_Flight_3472
Maybe the engine manufacturers are saying this type of engine failure isn’t possible on newer engines but I highly doubt any of them would make this claim about 20 year old engines.
Nearly impossible is not the same as entirely impossible.
Engines are supposed to be able to contain a fan blade failure. If an entire disc goes, there is no material (at least nothing light enough to allow a plane to still fly) that can contain it.
Even if the plane was 20 years old, there’s no guarantee the engine was. In this specific case, I’m not sure, but most planes have engine changes every few years, and the engines to through a separate maintenance/refurb program, so a 20-year-old plane could have a brand new engine, and a five-year-old plane could have a 20-year-old engine.
Regardless, incidents like this have even happened with newer engines. Qantas had a failure with a Rolls Royce engine on a brand-new A380 that actually damaged some control systems. The plane was out of service for a year or two after that. Air France had something similar to this happen with one of their A380s a few months ago (I think AF uses the GE/PW engine on the A380, but am not 100% sure of that).
These can be caused by premature corrosion, bad maintenance, or manufacturing flaws, and depending on what the specific cause is, can be impossible to detect. The best any manufacturer could do is say that such an event is extremely unlikely (on the order of one in several million or so), but nobody could responsibly say that such a failure is impossible.