I am a follower of Roshtein. From my perspective, I can say that he is a genius at his craft. I actually want to see more of him streaming. So I know the fact that there are various deals between casino and the streamer. Regardless, based from my conclusion everytime I watch his streams, I believe that the money that he is spending to play on Stake came from his own pocket.
I know it is hard to believe it. In my case, at first I did not believed that it is a "real" money, but then later on, I am convinced that the money is not fake. You can see how genuine his reactions are and how he behave on his next day stream almost everytime he had a big loss from his previous stream(s).
This guy, Roshtein, is different from other streamers.
You can't judge a content creator based on his reactions and how he behaves on stream, they know their work and they are pretty good at acting as well. If you are convinced that he uses his own money only because his reactions look genuine then you might have been convinced due to his acting. I'm not putting any blame on anyone, but that is what happens mostly.
Even if the streamers are using real money, that money is most probably sponsored by the casino itself since it is a pure promotion for them since there are thousands of people watching the stream and a lot of people follow that as well.