An optimistic projection, but the writing betrays hubris:
"With the Colossus, we have nothing more to fear."
"...securing peace in our time, and for generations to come."
"...having proven once and for all our technological and tactical superiority over our Great War enemies."
But all of these statements are absolutely true, as far as they (and the player!) know. The wording isn't terribly conservative, but it's not wrong.
And allied recon elements encountered the Sathanas in the very first sortie into the nebula - Kappa Wing, after all. They just didn't make it home.
Meaning the GTVA didn't know better. That they encountered it was irrevelant; we don't even known, in fact, that Kappa actually did
encounter the Sathanas. Some of us just think so. (While others note that the area within that close proximity to the node that Kappa would have patrolled would have been crossed and recrossed in the following weeks and engagements; Kappa would have almost certainly have to have subspace jumped to run into the Sathanas when nobody else does so. It's much more likely they met a Ravana, which is impressive enough. If you don't believe that, ask the Delacroix
. And Kappa didn't know the Colossus existed, a Ravana or three would have been bad enough to provoke that kind of panic.)
Moreover, 'acting in a manner totally consistent with the information available to them', in the face of a species that annihilated (at the very least) the Ancient Empire, is pretty much hubris on its own.
But how they did just that is a known quantity (shielding and the Lucifer's sheathe-shielding) and one the GTVA is capable of defending against.
You say that arrogance requires unfounded faith in superiority when there is evidence to the contrary, but the existence of those quotes above - combined with the assumption that the Shivans that the Ravana was the best they had, or that the Shivans had only one Sathanas - seems to be just that.
On the contrary; nothing you've posisted was contradictory to anything I said. That the Ravana was the best the Shivans had was never stated, and seems a poor assumption that nobody in the game actually made. They were basing their knowledge of the Shivans on current recon, which suggested only one destroyer in the immediate area, and the battle against the Lucifer fleet, which wasn't very large. Similarly, the conception of a single Sathanas seemed reasonable at the time; the ship represents a massive investment of resources and appears to occupy the fleet heirarchy posistion that the Lucifer did considering a serious lack of Shivan destroyer contacts during the war so far. The only reliable intel they had on Shivan fleet organization comes from the Great War, and it seemed to be borne out by recon and combat experience so far.