The term 'Mach Trim' is normally associated with conventional flight control systems which have to provide a feel force feedback to the pilot.
With increasing Mach No the aircraft might suffer reducing stickforce-speed relationship which defines stability; Mach Trim applies a corrective trim offset to mask the effects of this change.
The A320 experiences this change, but because the FBW design does not need any feel-force feedback to provide 'trim feel' awareness, any change or aerodynamic non-linearity is 'hidden', managed, within the FBW system.
Other aspects of trim are within the FBW computation, which enable 'equivalent manual' trim functions.