I guess that they will do the sportsbook inside their current website and not on a different website. We have seen plenty of casinos that started their own sportsbooks and they always did it on their own website and not on a different website.
There are pros and cons to run sportsbook on current website. Pros are it's more convenient and Rollbit will spend less budget to build up a page for their sportsbook, design, security, maintenance and other things.
Cons are they will have to figure out how to separate funds, secure funds, and protect reputation of their current casino. I strongly believe they can do it all well but I see some casinos decided to create new ones if they want to run with new products. If Rollbit team think they can launch their sportsbook on current casino, it's great.
It would not make any sense to get a new domain and start a new one and do the marketing all over again and try to just convince everyone to combine all of it together. I mean I get that it is not easy to build a whole new part to your already existing website, the coders will have a long and hard path to achieve it and it will not only take a long time to build it but long time to make sure it works properly. However, that is the only proper way to do it if you ask me.
Yeah promotion is another reason. It is challenge to build up traffic, reputation for a totally new casino. Rollbit team have to consider it as well. It is easy to launch a new casino and start from scratch. In long run, it is a good approach but in short run, it takes a lot of time and cost. Depends on what Rollbit team want, short term or long term, they will choose a best approach.