Comment by pdonis

> Which is caused by matter achieving a particular density.

More precisely, by an isolated blob of collapsing matter surrounded by vacuum achieving a particular density.

> to say they're not "made of matter" seems overly reductive here.

No, it isn't, it is making a very important point: that the matter that originally formed the hole is not there any more. The hole itself is vacuum. If you fall into it, you won't see any matter, even though the hole was originally formed by collapsing matter.

> The observable mechanics of a black hole are (probably) controlled by it's surface area and not it's volume.

Only in the sense that the horizon area is proportional to the square of the mass, whereas the volume is not even well-defined. The actual thing that is controlling the "observable mechanics" is the mass (and spin, and charge if present, but in any actual hole it probably won't be).