Nowhere does it say this. Maferath's barbarian horde dispersed because their leader was captured and executed and their leader made peace with the Imperium. There was never a battle where the Imperium defeated their enemies in the field. They were losing at the time of Maferath's betrayal of Andraste.
You overestimate the strength of blood mages. And of a the then slowly decaying Imperium.
"Their vulnerabilities exposed to the world, the severely weakened Imperium suffered the first Exalted Marches by the escaped slave Andraste and her husband Maferath who brought a barbarian army from Ferelden north to destroy the empire in 1020 TE. Slave revolts by elves enamored of Andraste's campaign exploded in cities everywhere, compounding the threat of the encroaching Fereldan armies. Finally the rebellion was put down and Andraste was publicly executed with her generals four years later"
From Dragon Age: Origins: Prima Official Game Guide Book:
http://dragonage.wik...cial_Game_Guide
Because it didn't fall, But that doesn't magically relegate the fact the Tribes and Clans got to its Gates, You can say that all the Imperium lost was the lands it conquered (80% or so of the known world) But when you are besieged and then sue for peace what does that message indicate besides a willingness to forgo hostilities for peace?
And Hardly it was a collection of the hill tribesmen of Fereldan and the Clans the War Chief could gather, it was a collection of forces far smaller then the one Tveinter set against them, As i said they maintained the largest military in the world by this point, Blight or no if it was merely a war of numbers, This little invasion wouldn't have achieved what dozens if not more failed to achieve.
The Imperium's borders being set for the reminder of time, Their Borders have not grown from that war til the dragon age, does that not indicate anything to you?
War ended on 1035 TE. In 1050-1120 TE, Rivain's war for independence from the Imperium begins, and distracts the Empire from reconquering the Free Marches.
With the founding of the Chantry in 1195 TE Southern Tevinter Imperium converted to Chantry. This is kinda related to real life event Roman Empire and Byzantine empire splitting.
There was other blights.
Then came the Qunari.
You see problem after problem.