However, this kind of advertisement will attract more players to deposit to their webaite if they see they are able to win huge like that of stremear.
This happens but not to all people. Smart gamblers will always think about what will actually happen or might happen if they deposited and play on the website, people who know how really gambling works will not go in and deposit right away just because they saw someone winning chunks of money on the said site.
The only question that I am confused with is that, what if the streamer won huge using that lent bank roll? Will they give him additional payment?
From what I've seen from many YouTubers that I watch who is being sponsored into various websites AFAIK they get to keep less than 5% of the amount they won. But surely it depends on the contract between the site owner and the content creator.