> Length of 6.2 m times 5 tonnes per cubic meter gives a sectional density of 31 tonnes per square meter, which is about 15 meters of dirt. So Newton's impact depth approximation would predict a penetration depth one fourth of the reported 60-meter depth.

This seems to assume that the weapon would penetrate until it displaced an equal amount of dirt by mass, which seems like nonsense. Why would that be the case?

You have the key phrase to Google right there in the text you quoted