He's right. The temptation to sin does not come from ourselves, it comes to us. We sin when and if we give in to that temptation. Had we not been tempted would we sin? No, not if we are true christians, we do not go around looking to sin. I want to do what's right, but sometimes I fail to- which makes me guilty of sin. Did I want to overeat delicious food and become overweight? No. I tried to resist, but my flesh got hungrier and hungrier. It's power became stronger than my self control. So i have to make my self control stronger and stronger until it overpowers my flesh.
If it was my root want to sin there would not exist a struggle not to sin. A caterpillar is a worm who does what they do- it's their nature. But once it becomes a butterfly it is no longer their nature to do what caterpillars do. It is now their nature to fly. I am not saying that when we sin we are not guilty of that sin, I'm saying that we do not desire to sin. We don't sit there and say "You know what, no temptation has come to me, but i think I'll go out and sin anyways."
We are talking about the source. Yes I want to taste delicious food, yes I want to indulge in overstuffing myself- but not if it means sinning against God by becoming a glutton. If not tempted to do so, and someone came to me and asked "Do you want to sin against God and become a glutton?" I would say no- it is not in my nature as a Christian to want to sin. But if you add the pull of physical hunger, the stress relief of comfort food, and the pleasure of taste- it is now hard for me not to, I still try to resist, but am not successful all the time (sometimes I am). So i need to get stronger at self control, to resist the devil- not to resist myself- if I wanted to sin i would not struggle so hard not to sin- I would just go about sinning freely without any resistance.