They didn't calculate it. It's the standard value from the scaling of force for a suddenly applied load, calculated decades earlier.

It's the theoretical maximum, and assumes an ideal undamped spring (which a building is not), so the real value will be less. You can see this experimentally here.

A weight, statically applied, is about 1.3 pounds. I raise it so it's still touching, but not loading, and release it. The peak weight (i.e. downward force) is somewhere over 2 (about 2.3, it's hard to see with the motion blur). So about 1.77, a bit less than 2 due to the damping in the scale.

2 is used conservatively large for safety reasons in design. But in this analysis, it's conservatively small. The weight of a falling floor is NOT suddenly applied from 0.0", it's falling. It's also not falling attached to a spring, or landing on a spring. The real forces involved are many times greater, and applied to floors that can't really deflect very much without failing.

The small 2x simply shows that global collapse was inevitable. The details of the collapse are more complicated.