I agree with you in the macro sense. But you have to have interacted with enough people to realize that motivation, while it doesn't change talent, can certainly impact the observed performance of talent.
Bulletin board material doesn't make a player better, but it definitely can help them show up and play their best. People have off days, with less energy or a "I'm not feeling it today". My life experience says that people use different coping techniques to find motivation to attempt to be at their best. Enough athletes talk about getting motivation from perceived slights for it to be a real factor in their thinking.
On this specific topic, I doubt Starks' comments are really going to have any bearing on Josh. He seems to have his motivation pretty figured out and this won't really impact the chip on his shoulder.