From the Go paper:

Conclusions The results of our study revealed that Baduk experts develop structural fronto–cingulo–striato–thalamic connectivity, as evidenced by increased FA of WM tracts, as compared with those of non-experts. These structures are associated with cognitive processes that include spatial perception, attention, working memory, executive control, and problem solving (Chen et al., 2003). In addition, the experts' increased FA in inferior temporal areas indicates that, unlike the situation in controls, task-specific templates had developed in experts' neural mechanisms, enabling the efficient operation of tasks related to playing Baduk (de Rover et al., 2008). Right hemispheric dominance in Baduk experts also suggests that the involved tasks are mainly spatial processes (Thomason et al., 2009). Therefore, this study demonstrated that brain training, such as that required to become an expert Baduk player, might cause structural changes in the brain that are particularly helpful with regard to engaging in such foundational tasks as learning, abstract reasoning, problemsolving, and self-control.

So, they got (? or chose the profession because they were?) good at a number of specific tasks that are components of IQ, but they aren't good at IQ measuring tasks overall.