Yes, the focus point metadata (and other Nikon-related metadata) is preserved in the DNG. At the technical level, vendor private data such as MakerNotes are basically TIFF-like tags that software can simply copy over from one container/file to another, even if the software doesn't understand the meaning of those tags (*). Just because other software does not show the metadata after conversion to DNG, it does not mean that the metadata does not exist.
(*) It would be like going to a warehouse filled with boxes and being asked to save or move those boxes. You can move those boxes from one warehouse to another even if you don't know or understand what's in those boxes.