The obvious answer is directoral (and authoral) license--made for a good scene in the film and the novel. But, as others have noted, the element of surprise was the major factor. Plus, Bruno Tattaglia didn't have to hold Luca's hand down in a struggle--just long enough for one of the guys to stick a knife in it. The shock and pain of that prevented Luca from shrugging off the garrote, which he might easily have done otherwise. I found it credible in its own way.