They left out a lot of math.
What is the area of the “average roof” in square ft?
To determine the snow load they need to take the weight of the and divided it the square feet.
Example: If the roof area is 1500sf then the snow load would be 10,000/1500 = 6.7PSF
Modern building code is 20-30PSF snow load
I should add that areas of heavy snow will have stricter snow load requirements to withstand heavier events