biased presumption that Civilization is a war game first and foremost.
The bias is concluding it isn't. The evidence that balance makes war dominant is overwhelming.
You have however, committed a logical fallacy by assuming that means war is the central gameplay of Civ 6.
I'm not sure which fallacy you believe I've committed since you didn't state one. However, not only can war win the game outright, none of the other victory conditions are remotely viable against opposition that is trying to achieve a victory condition unless the winning nation can defeat them in war. This is a matter of fact. No cities = none of the present victory conditions are possible.
You have no evidence to back up your claims that all other gaming conditions are secondary to war
I provided it already. You did not address it.
War can block every VC in Civ 1-6, and it can ensure victory even with domination/conquest turned off in Civ 1-6. When one plays against opponents that try, any time a player threatens to win he/she must defend, quite likely against multiple opponents. If the player still wins in this scenario, they can win domination and the pseudo victories are there to save time if the losers don't want to concede. If not, whatever pseudo victory condition pursued in its stead isn't happening; losing this dogpile war is game over.
Literally the only setting that changes this equation is "always peace", to force-remove the need to build units or defend yourself in any capacity. When this option is selected, the game is shallow indeed as players who get RNG'd into bad land have no recourse. You would also need doctored/mirrored maps just to get a decent non-war shake.
In other words, you have drastically alter the game rules and play under strict conditions just to make anything other than war the dominant consideration in this game, and when you do so a large portion of the game's technologies and civics become irrelevant, something that just doesn't happen to nearly the same degree if you were to disable religion or space.
Civilization is a game about Civilization, all it was, is and is to come.
What the game's own evidence supports is that Civilization is a 4x with a historical theme. Speaking of fallacies, I said that "the game's balance is centered around war", not that "civ 6 is a war game", so the comment about what war games allow doesn't do a lot for this discussion. You also didn't address the 8 vs 15 cities issue, or why the game needs an implementation of "your rules are not our rules" BS in a strategy title just to help the AI game throw.
Yes, peace is "an option" in civ. When someone else is threatening to win the game outright, it is also
objectively a false choice, one a competent player would only make when the pre-game settings enforce it. Not attempting to intercept another nation who will win is throwing, and failing to defend yourself is a loss in a way that failing to spread your faith is not.