I see that the below command line works fine:
./KStars.app/Contents/MacOS/kstars test.fit
It opens up KStar with the image shown.
Releasing a separate version of KStars just to get this to work seems silly
On macOS the opening of files is related to the UTI of the file according to: en.wikipedia.org/wiki/Uniform_Type_Identifier#Looking_up_a_UTI
Below you can see that a file saved by KStars differs in the UTI from a file that is saved by another program: Siril. Note that the file name extensions also differ by the extra s in .fits in KStars vs .fit in Siril. Both are "FITS file" though.