Untrue, you can learn timing, power spikes, efficient farming, get accustomed to ranges cooldowns and mana costs, learn different combos or small mechanics, test out item builds (if it doesn't work in Normals it probably wont work in Ranked), and generally just get a feel for the champion. Learning all of that is pretty important before taking a champion into Ranked. When I learn a new champion I do not look at KDA, I look at all the other stuff so I can get used to the champions abilities and build muscle memory as well as reduce the amount of time it takes to make a decision or play with them.
Basically if you go into Normals and look only at your KDA and whether you won or lost, you are learning the champion wrong. Learn the things about the champion that do not change from game to game, get accustomed to the unique aspects, then when you feel confident take it into Ranked. Playing a champion you do not know is putting yourself at a disadvantage, even Diamond players test out new champions in Normals to get a feel for them before taking them to Ranked. You will struggle to learn a new champion against people of your own skill level, they will take advantage of your lack of knowledge and abuse the shit out of it.