I wonder if this theory implies that the formation of black holes in our 3D universe would trigger the formation of 2D brane sub-universes? Otherwise, it would seem weird that 4D universe has this special propery, but others do not.

From a more philosophical perspective, even if this does explain our Universe, it really just shifts the goalposts for the question of how things came to be – how did that 4D universe come to exist?