> the mass density inside the event horizon

Is not a well-defined number. The number you are calculating assumes that the horizon is an ordinary 2-sphere enclosing an ordinary 3-ball [edited, see grandchild post] in Euclidean space, and divides the hole's mass by the calculated volume of that 3-ball. But the spacetime of a black hole doesn't work that way. The "space" inside the hole's horizon is not like any kind of ordinary space that your intuitions will work for.

Also, the interior of the hole is vacuum, so its mass is not something that's distributed throughout any kind of volume; it's just a geometric property of the spacetime.