Darktable uses exiv2 to extract the name of the lens. At least for canon mount it‘s done via a specific id as shown in my linked post. This is hardcoded in the exiv2 library but can be overridden using the ~/.exiv2 file.
You can determine the id via
exiv2 -pv --grep lens/i <file>
Maybe it’s not exactly the same for nikon mount, but similar.
Have a look at Lens not automatically selected - #10 by sotsoguk Theres a nikon lens used