If they plan to become known quickly through the forum, they have to spend money for ads including the forum ad slots.
i agree promotion is a nice way to have more user's and there's no shortcut to be popular exchange but having many kind of advertisement can help them to be known by user's.
I remember okex and kucoin do a promotion in social media such as facebook and sometimes they run a give away to attract new user's to registered and participate in thier exchange.
It's a process that they will go through just like the popular ones today. They went to the same process just like what this exchange is doing. And if the service was good and considerable, customers and traders will go here and there to trade and will spread the good news about them.
I also remember the time when kucoin was exclusively making promotions to attract customers through their exchange.