Actually, there is yet another tag…
[XMP] Camera Model Name : M8 Digital Camera
… which is the tag that has to be written to get PL to read the file.
In practice, explicitly writing only the xmp:model tag does not change the exif:model tag, whereas writing…
exiftool -model='M9 Digital Camera'
…writes to both at the same time. But you would need…
exiftool -model='M9 Digital Camera' -uniquecameramodel='M9 Digital Camera'
… to maintain consistency of all tags that refer to the camera name.