Well...this brings up a fascinating 'chicken or the egg' question as relates to the video and the timeline they propose.
Yeah, but if I had to guess, I would say that the BH forms early on and creates a huge gravity well which then causes more matter to be drawn into the area, whch then creates an even bigger gravity well - so on and so on... A small cluster of stars with one really big one that collapses and becomes a BH can then become an even larger gravity well that draws in more and more stars as the eons pass.