I have to agree. That quote sounds like a precise description of M-sync flash. If you look at some old shutters like this Ilex pictured on S. K. Grimes' website:
you'll see there are numbers for different flash sync settings. These refer to the number of milliseconds in advance of the full opening of the shutter required for synchronizing different types of flashbulbs. For X-sync, one sets the shutter on the "0" setting, so that the strobe fires precisely when the shutter is wide open, not before, because the peak is virtually instantaneous. The only relevant concern here is for studio flash units, which might actually have a longer duration at full power than the shutter speed.