This is not actually a density threshold; the more mass you have, the less density you need/get withing the event horizon, meaning bigger black hole => less average mass density ("Schwarzschild density").
And this Schwarzschild density is comparable to the mass density of neutron stars (or atomic nuclei) for small black holes, and can be as low as the density of water (!!) for the supermassive ones.
Right, makes sense, it's all about it causing a sufficiently "steep" space curvature and/or over a long enough distance, to offset the speed of light trying to escape. Of course, high density is not the only way to have that effect. Thanks for the insight!
And this Schwarzschild density is comparable to the mass density of neutron stars (or atomic nuclei) for small black holes, and can be as low as the density of water (!!) for the supermassive ones.