Yeah you don’t really need that much detail to get a significant advantage. Even just knowing x,y,z topics weren’t tested but a,b,c were will improve someone’s chances. They could then double their efforts on a,b,c and not waste time practicing x,y,z.
Those types of things are also more likely for someone to let slip to a friend too. “Oh man they had some really tough questions on b.” Or, “I’m surprised they didn’t cover x at all.” Still, I wouldn’t expect there to be enough people doing this to impact the pass mark. If you learn the material well enough, you will pass regardless of what other candidates are doing.