The MOV is being installed across Line and Neutral which is correct. Since this is a single phase socket array, all the 3 (Line Neutral and Ground) are shorted together via the copper busbar. Therefore the potential differences between the sockets should be negligible.

More expensive surge arrestors will have MOV across line-line, line-neutral, neutral-ground and line-ground. Obviously this Mr DIY is just a extra precaution instead of full protection~ better than nothing i suppose haha