The 12ga reference may be for connections to the solenoid actuating circuit (2 circuits bottom right M-Unit that activate solenoid). The circuit through solenoid from battery to starter needs 6ga (8ga-4ga depending on length). Most MC starters are in the 100amp neighborhoodCheers JP.
Revival also state (on a motorbike) 12awg is more than enough for battery connection; not 6awg states in manual.