means center and () means "as center". Like you suggested,
>
> active all around the world, with South-East Asia as center of activity
Also, should not refer to , stands for many, in the sense of many people.
My rough translation would be
>
> Moreover, considering the fact that many of the so-called (Chinese merchants overseas force) are active all around the world, South-East Asia being the heart of their activity, one can understand the Fujian people's power that comes from going overseas.