Bullshit.
George Karl HIMSELF said he didn't press the issue.
Melo isn't going to become an all NBA defender, not now, not ever. Hell he may not have become one if Karl had him develop that side. But with proper coaching team defense can get better, and as an individual defender Melo may see improvement.
When Lebron came into the league he wasn't a good defender. It really wasn't until Mike Brown was the head coach and pretty much focused on defense primarily did we see Lebron start to develop as a defender. And it started with an improved team defense and strategy.
It's rare when players have a mix of top level defense and top level offense. Often you see guys who are great defenders and just average offensive players. They know what they have to do to keep a job in the league.
It's not too late for him to become a better defender. But at this point it's going to take a coach with a lot of respect to get that out of him.