Has the dude in question conquered half a continent in the recent past? Because I feel like that would impact upon my decision.
That's OK. Not everyone has to be the type of person who'd fight to defend what's theirs. GL with that.
His "half a continent" worth of army was still smaller than the last fief's.
It's certainly smaller than the Khan's
whole continent!
If you had a bigger army than the guy on your doorstep with his illegitimate claim, demanding you give him your lands and title, you would treat him with respect & just turn your stuff over? Really!?
I wouldn't. I'd treat him with the same contempt most of them do.
All of the rulers have shown a willingness to fight in defence of their land & title.
If they fight & win, they get a bonus from the Khan & possibly added status in the Khanate.
If they fight & lose, they work for the usurper, but they don't die. Not a single one of the other ex-rulers is dead. If the Khan takes the land back, they get rescued.
If they don't fight & just roll over, they get the same as if they fought & lost. Except the Khan will kill them for it, when she takes the land back with her force that's twice the size of the usurper's.
So, I think it's pretty obvious why they could want to fight, instead of rolling over & submitting.
Never mind the simple fact that they genuinely think they can win, due to superior numbers.
I certainly don't see any reason for them to treat him with any respect, given what he's demanding.