The black hole in question is eating matter near the rate of what is theorized to be the limit of how fast a black hole can consume matter. We've really never seen anything like it. ~875,000,000 is very young for such a structure to be so big in a universe that young. Anyway, when it is said that it is the brightest object in that part and time of the universe, we are speaking of the "extra bright" event horizon: the point where matter is super heated before tumbling into the singularity itself. Since it's all one dense gravitationally bound structure, the event horizon is part of the whole. The great part of this discovery is that it allows us to take a peek at ratios of elements such as hydrogen and helium relative to heavier elements in a still pretty young universe.