That is correct, it will only alter BitmapSource (raster) images or BitmapSource image portions that are nested in DrawingVisuals. The latter allows you to have a raster image for say a highlight pen and then use a vector portion to render the actual pen color. When that converts to monochrome, the highlight pen raster image portion will be pure white but the pen color will still show through for the vector portion.
We are updating our documentation for the next maintenance release to clarify that it's for BitmapSource images only.
For QAT controls that use vector images, you'd have to supply your own monochrome versions.
If you have any suggestions in this area, please feel free to post them.