Not necessarily. While they can be, it varies greatly between manufacturers. If you are looking for a specific density of brush, I would recommend looking at reviews and seeing what other members say about it first.
No. Density has to do with how many hairs are stuffed into a handle. More hairs = more density. There are lots of examples of brushes with small knots that are much more dense than brushes with large knots. A more dense knot usually has more backbone than a comparable brush with less density, but backbone can also be influenced by the stiffness of the particular hairs used in a knot.