Darwin has easier method for clonefile checking.
It seems like our compatibility testing method might be easier on macOS:
Not all volumes support clonefile(). A volume can be tested for clonefile() support by using getattrlist(2) to get the volume capabili- ties attribute ATTR_VOL_CAPABILITIES, and then testing the VOL_CAP_INT_CLONE flag.
So one might want to implement this natively instead of naively :-)