That's the stupidest thing I've heard in awhile. The bicep is 2 muscles. While you can't isolate and specifically work just one of them, you can adjust your grip to emphasize or target one of them. Just for shits and grins, next time you go to the gym, grab a cambered bar and load it with as much weight as you would normally do a curl with. Grip the bar close-grip with your hands close enough they're touching each other, and do a curl. If what you're saying is correct, it will be no harder and no easier than any other curl you've ever done. But I'm betting you're wrong and not only is it more difficult, you'll feel a very hard contraction on the outside bicep.If you could target the bicep the way you say, then you should also be able to flex all the different parts of that muscle separately.