Nor the Manga nor the Artbook said that GER needs to hit to kill the opponent to send it to the Loop but just struck it. And struck means hit.
Not all the stands which have contact-based abilities always use them at the moment when they touch a random thing. For example, Gold Experience, who's known for turning the things it touches in animals, doesn't always do that when it hits something. Or Crazy Diamond, who's known for repairing the things it breaks, dosen't automatically repair the things as it touches them. GER is the same example, especially when Giorno likes to kill the opponents in a painful death like with Cioccolata, Melone or Ghiaccio. So him using 2 MUDA Rushes against Diavolo doesn't debunk him needing just to hit once to send to the Death Loop, but it was just Giorno who wanted to torture Diavolo a bit before finishing him like he usually does with the previous opponents.
Basically peeps who say GER needs to kill to do the death loop contradicts WoG (this was written in JoJo A-GOGO, idk the spelling) and previous parts.