Bit late to the party, but I can shed some light on this. Since a real camera has the shutter open for some duration, any moving light will smear across the sensor. If you similarly "smear" a path traced object by stochastically randomizing the position of the object while the path tracer is gathering samples for the frame, you get exact physically plausible motion blur without having discrete "ghosts" corresponding to sampled subframes.
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