That's an impressive swimming pool ;-)
I don't think that's technically true. You could build a bunch of catapults on the edge of the sphere, and when they all launch rocks at the center of the sphere, they would eventually form a black hole. The catapults could be arbitrarily far from each other as the radius of the sphere increases, such that they would not really do much to each other gravitationally. You'd just have to wait a real long time for the rocks to hit the middle.
> Building black holes with anything other than stars or giant gas clouds (in the early universe) turns out to be hard;
Well yes, galaxy-sized intelligently designed structures don't really happen.
I take your point however, that you could coordinate in some way separated masses, but at some point you'd probably run into issues with the aforementioned galactic-scale of engineering.
I found it extremely amazing as well! :p