The standard Java 2D library has been use to generate image (http://en.wikipedia.org/wiki/Java_2D).
For SVG output, a custom class SvgGraphics has been written : it's not a very difficult job.
However, even for class diagram, GraphViz is not use to generate the image. GraphViz is only used to compute node position.
The work is done in net.sourceforge.plantuml.svek package.
But as you has noticed, documentation about this is not complete yet :-)