How do you know that? Citations, please.
I don't remember exactly where I read it, but I know it to be true
Nothing on their UESP page, but the lack of any mention of wars before the Night of Tears http://www.uesp.net/wiki/Lore:History is conclusive evidence that no war major enough to be recorded occured, leaving their troops untested against the admittedly accomplished Atmoran Nords who had come from a country succumbing to a terrible winter and a collapsing society structure
Atmorra is colder than Skyrim and we see Ysgramor personally in the game. He's a Nord.
It is colder now as climate change or something similar made it colder, at the time of Ysgramor it wasn't as cold.
Skyrim has also advanced since the time the Nords defeater the Falmer.
Technologically maybe, but culturally they have gone from a force that could manage invasions on multiple fronts to a province that can't effectively secede from the remains of a collapsed Empire.