So? Fact still remains that Calian believed in Duncan's word enough to gather said armies in the first place and stay there sans evidence of an actual Blight.
There's evidence of a large darkspawn horde gathering in the south. The evidence is the horde itself and the tainted land.
If there's to be evidence of a blight, it requires an archdemon. Without an archdemon, there is no blight. Unless Duncan and the Wardens, or the scouts were able to pinpoint the archdemon, all the leaders knew for sure was that there was a horde of darkspawn.
That doesn't mean a blight off the bat. Even Alistair points it out that most think it's just an unusually large darkspawn raid because no one saw the archdemon.
And tracking down the archdemon and gathering evidence that it's a blight is the very reason Duncan went to Orzammar in the Dwarven Noble origin.
He says it to the Warden after talking to Cailan. "I know that an archdemon is behind this, but I cannot ask the King to act soley on my feelilngs."
The Horde itself was real and could be pointed to, and that is a very real threat worth calling all of Ferelden's armies together for, and everyone saw the darkspawn. No one saw an archdemon, except in Warden dreams.