This failure is the stuff of nightmares for sure. There's sound reasoning behind a big bung, but it's not everything. The standard Cannondale carbon steerer bung on my 2021 supersix is small, maybe 20mm of internal reinforcement - I don't think Cannondale would be taking chances.
The rust and dirt inside the tube here is super dodgy. Has the aluminum bug and carbon tube been compromised after repeated saturations of sweat from use on a turbo trainer?
The guy also has a ridiculous amount of spacers above the stem, but tbh, it looks like the bung was still at the right depth to reinforce?