Just a couple of minor problems. Their stomach contents were still undigested and their flesh was flash-frozen, some of the meat was fed to the dogs of the explorers.
I don’t think that precludes it from having died in a river and shortly thereafter frozen and covered with silt. Floating 400 miles north could have taken the carcass from a cold place to a super cold place.
I saw a theory many years ago that volcanic activity blasted huge quantities of gases into the upper atmosphere. When the gases reached the edge of space and were supercooled, being heavier they fell to earth in large "blobs". That accounted for the instant deaths and flash-freezing of so many critters in Siberia.
A sudden super cooling of the atmosphere would have caused flash freezing and glaciation in many Northern area and Vast rains to fall all over the Earth. The great Flood!