It doesn't have to be uniform, no. And in general it's not going to be. The article this is from was just noting that, if you put X much stuff within a sphere of size Y or less, then it will be a black hole -- regardless of how you distribute it. And that equation relating Y to X is such that the required average density (computed by taking the volume of Y) goes down -- so that, no matter how low a density you want to talk about, if you fill a large enough sphere Y with it, there will be enough mass of it to be a black hole.