It doesn't really make sense to ask KYC for big winnings since it can easily be verified if the win turned out fraud or cheat.
The KYC instead should be mandated if the account was triggered by the security and something suspicious is being suspected.
For example, a big amount was deposited on that account, and then try to withdraw it without wagering. That is an obvious attempt that something is taking place.
I think the casino will not allow you to just deposit and withdraw without wagering, they put something in their system that you have to wager x1 or something like that because it's obvious that you might be using their casino to mix coins and thus it could come from a tainted and bad source.
But in any case, I would agree that the casino should ask for KYC before any withdrawal, not just huge amount of winnings.
Because asking it late will sometimes cause issues with gamblers who doesn't cheat and take advantage of some exploits in their system.