Why is bipolar EMG better than monopolar at reducing (common mode) noises?
According to the link you provided, as far as my understanding goes about it, you have interpreted it wrong.



EMG_mono = k*(V1 - Vref) and EMG_bi = k*((V1 - V2) - Vref)



As explained in various answers here the bipolar electrodes further help to reduce the common noise in the two electrodes (which is basically the line noise picked up by the body), which the monopolar fails to do so since the electrodes are far apart and the (picked up signal) noise may be different which degrades the SNR. Since the bipolar electrodes are closer to each other (1 - 2 cm apart) the noise running on both the electrodes is approximately the same, and hence can be effectively removed before subtracting it with Vref.