Reports like this make me think I need to get into the habit of doing pre-ride safety checks of my bikes. I happily spend money on and use a torque wrench for initial installation, but I suspect that a pre-ride check is as important as correctly installing a steerer and stem etc., if not not more important. The guidance I’ve read suggests holding the front wheel between your knees trying to turn the handlebars to check for any movement in the steerer/stem.
I guess there will always remain the potential for sudden catastrophic failure without any advance warning of a safety critical component, which seems to have been the case here, but I think that very often there are indications that something is wrong, which a pre-ride check will hopefully identify.
Based on some of the comments on this old CTC forum thread about an aluminium steerer failing, the potential exists for all types of steerer to fail dangerously, even old fashioned threaded steel steerers.