In the video that you posted it doesn’t look like he is pulling a heavy weight to me. Look how it’s moving around. Regardless, and I’m speaking on limited knowledge, sprinting with a sled (up to 30m) makes make sense because as Charlie Francis said; sprinting alone is a body weight exercise.
So think about it; how much stronger could you get in the gym if you could only use body weight exercises? From what I can see the sled pulls is working because once Asafa gets to about the 30-35m the race is pretty much over for the rest of the field.
More specifically what I’m trying to say is, the sled training/weight training has to be key ingredient to producing the powerful block clearance along with the powerful few first steps to separate yourself from the rest of the pack early.