Todd mentions he's been doing it that way for nearly 30 years, since receiving a tech bulletin from Gottlieb to add the single extra ground wire in the 80s. I asked him about it in the video comments once and he said he'd never had a problem in all this time. He also mentions on several occasions that he knows other people put lots of grounds all over the game and that he believes it to just be overkill.
I remain unsure who I believe. Clay and Todd both have years of experience. Todd has seemingly sold and operated every solid state game in existence. But Clay has a lot of technical electrical understanding. Who wants to buy two identical System 80 games, put one ground in one and a ton in the other, then burn them in for weeks and see if the one wire version fails?