I have the feeling the author has never seen a real avalanche... The ones that are that fast are usually very powdery, look like clouds, can catapult you far away, and the ones that hit that hard like a train or truck are often slower, but very massive and big and once they stop, they are like concrete.
And yes, I already saw avalanches in nature, still moving in some cases, but in most, they came down hours or days earlier.