Ok, I guess it depends on how long the scourge lasted... I vaguely recall it was more than a few generations, and with long lived races like elves and dwarfs to consider that would put the ED line on that graph a little nearer the end of the cycle wouldn't it?
EDIT: Dug out my copy of Earthdawn, apparently the scourge lasted approx. 400 years from 1008 to 1420 in the Throalic calendar so as has been pointed out in other threads, the appearance of the Horrors in SR (not that they have exactly) is ridiculously early, but that's the
beauty of mass scale blood magic I guess