A streamer winning huge amount is very hard to believe these days since most of them is known for just doing acting because they don’t use their real money as bankroll when gambling.
Agreed. Honestly, I don't believe any streamer/influencer winning big no matter how good they act. Newbies posting proper evidence of their wins are far more trustworthy in comparison.You need to be quite gullible/naive to believe those big wins. It's a pretty clever way to market crypto gambling sites though.
They can gamble all the way and would finally win in the end. However, it could be that they have an agreement with the casino like the account is being tipped and that will be use for gambling purposes. This is for the show, because it's pretty obvious, you promote the site, so you need to attract the audience and there's only way of doing that, and that is by "winning".
This is true and exposed already by influencers on social media. They are using a balance which is their salary by promoting the casino on their stream but they need to wager it before they can withdraw so it’s a plus if they manage to have a profit but most of the time their balance is not their original but rather from the casino.
I believe a streamer on their right mind will not casually spend a grand on buy bonus while they knew that it burns a lot when they lose.
I agree on @Haunebu that a transparent post of a newbie showing an actual game results that can be verify is much more believable than this kind of win.
There's actually no problem with the promotion, as we can really win a game as long as the casino is not rigged. But for us, it's just a matter of when because gamblers have limited funds, unlike these influencers who have unlimited funds, as those will easily be topped up by the casinos to ensure they keep taking action. But that money is just for the promotion; they'll make a post once they hit a big win that could surely entice their followers. There's nothing illegal about that; it's just how promotions have been done. What we need to question is the casino's reputation because if it doesn't have a good one, then we should stay away automatically.
I suppose they open predictions day or two before the event. While selecting, they offer various options and % chance that the selected result will happen. Rather cool option; very helpful for those who are not familiar with fighters and dont want to make it random. 
And I can be wrong as well. I think I'm getting something wrong about this promo by Duelbits. Every time I go to the site and see this
