It's not relevant to this approaching army thing though. It doesn't take an army to attack a small village of unarmed citizens. A dozen, two dozen maybe.
Which gives the mages justification to accept Tevinter's help as they needed as many armed forces as possible.
If the Tevinter showed up and the attack never came, then Fiona looks like an idiot, but if the attack did come, even if they aren't comfortable with Tevinter, many mages would feel safer if it was Tevinter that drove off the templars in exchange for servitude.
The whole point of Fiona's justification was they needed help from an impending templar attack. If the attack happened, that gives Alexius and the Venatori political weight in Redcliff.
I'm assuming those were the same "templars" that ignored the call to Val Royeux. Sort of like those apostates that're hiding in that cave except when they were rampaging across the Hinterlands because "LOLs POWAAA!!"
Did you read those letters? Both groups were bonkers.
I know and established that in a previous post. The point I'm making is that if the town of Redcliff was actually attacked by these templars, and people won't differentiate between the templars who followed the Envy Demon/Lucius or the templars who ignored orders to go to Val Royeaux and stayed to fight, the mages in town would feel pushed into a corner, they need extra bodies and a way out of almost certain death for themselves and the people in the town. Then, *trumpet sounds* Alexius comes in and offers everything they need in exchange for service.
And based on the elven healer in Redcliff treating people exclusively in town until she's convinced to leave as part of a quest, I think that the town actually was attacked by those bonker templars, and this attack is what gave Alexius and the Venatori the political weight they had when we bumped into them in-game.