If you build 3.8.1 from source, then you need to change both adobe_coeff.c and cameras.xml. Otherwise you cannot add support to already built 3.8.1.
For master, you can simply change your cameras.xml even after the build, but with the addition of color matrices, like this: Nikon Z 9 support (lossless only) by kmilos · Pull Request #347 · darktable-org/rawspeed · GitHub
Note that you only need the “14bit-compressed” section in any case, there is no uncompressed mode on the Z 9.