User blog comment:Spritydove/Deeper Thought into the Civil War/@comment-80.216.219.47-20120701003627/@comment-80.216.219.47-20120705221634

Nothing on these pages says the info about it came from Ysgramor:

http://elderscrolls.wikia.com/wiki/Night_of_Tears

http://www.uesp.net/wiki/Lore:Night_of_Tears

However they do say that maybe the Nords had found something the Snow elves wanted, a magical artifact.

So perchance it did not happen because humans became too many for the elves liking, but that does not mean the butchery in question did not happen.