Think about this:
Assume a squishy cotton ball 500km in diameter (for some reason, it hasn’t collapsed under it’s own gravity, and keeps its cottony fluff), speeding toward the Earth at 50km/s.
Now, a cotton ball doesn’t transmit force throughout it’s bulk like a rigid object (e.g. a big rock). Imagine a column of stone, and you’re on the bottom of it. On top of it, somebody hits the column with a big mallet. You will feel almost the full force of that mallet-blow at the speed of sound through that column, which is likely to be very high; virtually in an instant. Do the same with a column of fluffy cotton. You probably won’t feel anything, if the column is tall enough.
Given that compression waves are transmitted so poorly through fluffy cotton, let’s imagine our 500km-across cotton ball, moving 50km/sec. When the leading edge of that ball hits the Earth, it’s trailing edge will be 500km above the surface, oblivious to the impact. It will continue to travel at 50km/sec for whatever time it takes to compress the cotton sufficiently to decelerate. If you figure the cotton can be compressed completely flat without resistance, it will take ten seconds for the trailing edge of the ball to join the matter of the leading edge on the ground. Obviously, cotton is not infinitely compressable, but at what point does the force of some substantial chunk of the trailing portion of the cotton ball meet an equal resistance due to compression of the cotton as the ball plows into the Earth’s surface?
I have no idea, but I bet it’s not nearly as quickly as a solid rock asteroid would transmit force up through it’s bulk from leading-to-trailing edge in an analgous situation. The way a fluffy cotton ball of that size will dissipate energy has got to be different than a solid rock of equal mass.