Source of income is like the document that is request for when a gambler wants to pass a level 4 kyc verification on Stake.com, so yeah, proving source of funds to casinos is nothing new, the only problem there is that, even before the casino allows a player deposit such a huge amount of money, the casinos should have asked the gambler or user to pass all level verification which includes proofing the source through which all the fund the gambler will be using to gamble in the casino comes from.
That would be a fair practice, but everyone does exactly the opposite because they don't mind someone losing $100k gambling, but at the same time they resist paying out winnings and demand proof of where the money came from. It seems quite hypocritical to me, because if you are ready to accept a deposit without any conditions, then you should have the same standards when paying out winnings.
What they are doing right now is trying to cheat the player, but such a matter can't be handled carelessly, the user should stand his ground, and if he has the capability, he should seek legal help, this is if the casinos hasn't paid him already.
I wouldn't call it cheating, but adjusting the rules as it suits them in a certain situation. The casino has the right to request KYC or the origin of the deposit at any time, and I think that they are legally covered there, and every player who won in a legal way and with money that can prove the origin will get their winnings sooner or later. Of course, unless it's a scam casino, but that's a completely different story.