That's some unsettling footage.
Those cars the FedEx truck hit were essentially a ramp; I'd be surprised if those folks survived.
Not having been there, it's hard to say what might have prevented or mitigated that, but sudden ice is obviously a nasty thing.
On non-winter tires, sailing along, and then hitting the brakes...trajectory & time are pretty much running things.
My brother said my niece would have been there except she had the day off.