The speed to the line's movement -- the speed of the induced downdraft, the strength of the gusts at the ground and the following short, but devasting winds, etc.

it's more that just a squall line of thunderstorms. It really is a different, linked structure, moving at (in this case) about 50-70 miles per hour over hundreds of miles without breaking-up like normal "thunderstorms" the pop-up, intensify, weaken, etc., etc.