Thing is, the pressure is only inwards from the coil sides, not from the top and bottom. The coin doesn't necessarily become denser, just smaller in diameter. The coin gets thicker to occupy the same volume. That's because the force is not exerted from all sides. Just from around it where the coil is, not top and bottom. In order for it to reach critical mass, there would have to be some very scientific engineering done just to contain the force and cause the item inside to reach that extreme. Look at the destruction that occurred in the blast box just in their little experiment, and that didn't even significantly change the density of the coin. Imagine what it would take to change the density of the coin, and do it so much that it reached a critical point. Lead, gold, or most any other dense metal would blow to powder before reaching a critical state in the device they used.