The issue is that we really don't know how powerful or not-powerful the Aldmeri Dominion is. Maybe they are as big a threat as they're made out to be. Maybe Titus Mede signed the W-G Concordat because there really was no other alternative. Maybe he signed it because he was tired of the war and just wanted to declare peace. We don't really know.
We know that the Dominion of 30 years ago were powerful enough to nearly eradicate the Blades and take, but not hold onto, the Imperial City, but we also know that the Dominion of 20 years ago was either not strong enough or not motivated enough to hold onto whatever portion of Hammerfell that was ceded to them.
A crapload of Skyrim, especially as pertains to the civil war, can easily be viewed as opinions or propaganda, even if it's in a book. It seems to be largely a matter of who you, the player, choose to trust and distrust.
And ultimately, it boils down to what Bethesda wants to happen; all else is essentially fanfiction. They want a Thalmor victory no matter what happens to SKyrim, there'll be a Thalmor victory. They want a Thalmor defeat no matter what happens to Skyrim, the Thalmor will be defeated. They want Ulfric to win and ally with Hammerfell, then he'll ally with Hammerfell. They want Ulfric to win and not ally with Hammerfell, then he'll not ally with Hammerfell. They want the Dragonborn to go down to Summerset Isle and destroy the Thalmor single-handedly, that'll happen.