Roleplaying and optimization are not connected. Trying to claim one reduces the other is a false dilemma.
I do believe it to be false, I believe that you have a false dichotomy. A good roleplayer needs to be able to optimize, because the unspoken question is, optimize for what?
Lets say that my character concept is that of Inigo Montoya. A six fingered man killed my father and left me scarred. I've spent my life becoming a champion fencer, and will one day have my revenge. Lots of roleplay-oriented hooks in there, but being really awesome at swordfighting is part of the character.
It doesn't even have to be combat. If I'm supposed to be really good at baking pies, or whatever, I need to know how to use the system to make it reflect my choices.
I think D&D is vulnerable to these kinds of beliefs for a couple of reasons. One, it doesn't tend to model much for roleplaying purposes. Take the Inigo Montoya example. I can make him fight with a sword, and perhaps skilled as a swordsmith depending on edition. But I can't make him hate the six-fingered man, have scars on his face, or be motivated by revenge the way I can in most other games. And starting with 3.0, the complexity of the rules to make someone really good at fighting are so in depth that they attract a lot of attention.