>There is no valid general conversion between speed and Mach number, and things like Mach number relative to the ground is nonsensical.

Sure, if you also want to say water doesn't freeze at 0C or boil at 100C. In practice if an author casually leaves out the variables needed to calculate ground speed from mach or airspeed, it usually means they're doing engineering calculations set at the ISA standard day model (sea level, 15C, no humidity).