You may be right that the higher peaks are the reason sinewave 120Vac RMS is a heftier shock risk. Or it might also be the fact of being AC, in the sense that a square wave inverter output with 120V peak and RMS voltage maybe is more dangerous than 120Vdc. I don't know if it is or not. I could make up reasons that it might be, like maybe it's harder for neurons to adapt to, or maybe electrolyte depletion near the electrodes increases the body's resistance, but I don't have any evidence.

There really are some shock risks that result from your body being heated as a resistive load, like internal burns, but those are not at the top of the list, especially at only 120Vac.