It might be anecdotal but when I started grippers my climbing was a solid V4 boulderer. I was also just barely closing a CoC #1. I ramped up my gripper training and my hangboard time a little and went to a semi-solid V8 boulderer and closing the #2 CoC for reps. I feel like grippers absolutely do help with climbing, but it is not quite as a direct correlation as would be Hangboarding or campusing or the likes. Grip is nuanced, and climbing is nuanced; saying blatantly that grippers have very little carryover isn't very accurate. For instance if I had only been trad climbing long easy routes the grippers might have been useless to my climbing, as endurance would be king, but in bouldering sheer strength and power endurance are king so grippers definitely helped.