Yes, you're wrong.
All IVs matter, whether defending or attacking.
Defense is responsible for reducing damage that you will take (HP lost). Attack will increase damage. HP is your survival time, which is enhanced through damage reduction by Def.
It is usually advisable to look at base stats and balance the weakest one through higher IV. For Tyranitar that's HP, but Defense isn't that much higher, so you should consider them both.
Your Rhydon will be a pretty good attacker. Rhydons problem are his moves anyway. And your Rhydon will faint a little quicker than a better IV one.
