I think I sort of see what CO is trying to say although he's sort of doing it lazily.
There is a certain point where a pitcher becomes a defender, when the ball is in play. However, when evaluating a pitcher's skill, you look at the things he has much more control over, such as walks, strikeouts and home runs given up (those balls are obviously not playable). So what he's trying to say, albeit poorly, is that you want to look at the pitcher's fielding-independent peripherals.
Pitchers are definitely still fielders, as they help protect the middle of the infield on comebackers and can check baserunners and even pick them off. That part of the game is important. What they get paid for, though, is their ability to generate as many outs by themselves as they can. So you look at guys who can keep the balls on the ground, who can get the most whiffs, and who don't like putting guys on base for free.
Once the ball is in play, the pitcher has almost no control over what happens to it. If it's on the ground or popped up, it'll probably find its way to an infielder, but that's about the extent of the control. However, a lot of a pitcher's worth is still dependent on how many runs he can prevent, so even if a pitcher's FIP or SIERA or whatever looks great, he can still be pure shit because when he's not striking them out, he's serving up doubles in the gap or whatever despite having a solid defense behind him. So just like everything else, you can't evaluate a pitcher based solely off of FIP or luck...you have to look at the whole picture.