Turcopoles were mostly local natives, some of whom were Christian but many were not.
Okay, what percentage are we talking? And out of that percentage, how much of the Crusader armies did they compose?
Looking at Hattin
https://en.wikipedia.org/wiki/Battle_of_Hattin it looks like there were only 500 Turcopoles out of ~20,000 soldiers. If even half of the Turcopoles were Christians, that means that Muslim native people only comprised ~0.5% of the Crusader army then.
Saladin's Christian hostages and prisoners
www.academia.edu
"As we have seen, many (though not all) Turcopoles were recruited from Christian-ized Arabs and Turks."
Why would you switch from considering game mechanics in abstract, then using them as direct evidence, then go back to abstraction again? Especially when we know the reason that the culture penalty was introduced --
to help support rebellions as a specific gameplay mechanic and assist in keeping snowballing in check.
Your original argument for culture-blended AI armies was the lore, arguing that cultures are blended in the lore: "As far as Bannerlord goes, these guys all speak the same language, have the same religion (none) and share a similar culture."
My response is that cultures are not blended in the lore/setting and are
also not blended in the gameplay mechanics.
That's why.
The language thing is abstracted because it
has to be in pretty much any game that deals with language - it would be extremely annoying to have to learn 6 different languages or get translators to be able to play a video game. Everything else in the game mechanically and in-lore shows that Calradia's cultures are very different and separate, "freeze-dried and plopped into the game as monolithic entities" as someone once said. Therefore the language not having a unique mechanic is solely an abstraction for the purposes of fun (or, more likely just an oversight).
If culture penalty was solely an anti-snowballing measure then it wouldn't be called culture penalty, it would be called "new ruler confusion" or something like that. But it isn't. Hiring a Battanian governor is enough to placate a Battanian town, who otherwise hate you for being Vlandian.
At any rate, Khuzaits are not a loose confederation and neither are Aserai.
From ingame Aserai encyclopedia:
"Today, with the waning of the empire, the Aserai have
agreed to form a
confederacy under a sultan chosen from the richest of the clans, the Banu Hulyan. But everyone knows the dance has only temporarily been stopped, and at the right moment it will begin again."
In other words, the Aserai are a loose plutocratic confederacy held together by popular consent.
From Khuzait ingame encyclopedia:
"He imposed discipline on the ruling clans, forcing them to ride to war on his command instead of simply whenever they wished. But after his death, the spirit of unity that he inspired was lost. His descendants still rule the khanate. But some of the other Khuzait leaders chafe under the authority of the current khan, Monchug, and others dream of becoming khan themselves."
From Khuzait devblog:
"Urkhun died, and though his descendants still rule the Khuzait Khanate, the other clans feel that they should be the ones to raise the nine-horsetail banner that symbolizes the supreme authority."
In other words, the Khuzait khanate that was only kept together
solely by the threat of force, not arguments to legal authority or claims to some kind of right (like Aserai/N.Empire/S.Empire) or even popular consent. Perhaps they are not literally a confederacy, but they are certainly distinct from most other factions in that regard.
Point is, the rulership systems are different, and in addition to everything else I said, it is pretty clear that the game's cultures are very distinct.