Read the whole article. It gives you a breakdown of what they used to project that. FG% isn't the only thing that makes someone valuable.
Team USA Carmelo is an at least Gamma-tier player. The Knicks Melo though? He's just above average starter.
There's an explanation at the bottom of the chart "* Player was acquired midseason. His +/- total reflects performance for entire season, and not just for the acquiring club." Drexler was the alpha in Portland playing at a very high level before he was traded to Houston.
How do you know CP3's past CPM? Only Nate Silver has an access to such data. Nevertheless, if you're talking about prime CP3 and prime Harden, there's argument to be made for CP3 being alpha over Harden since he plays both sides of the ball at an elite level, as well as him being a vocal leader. But a 32 year old CP3 is not gonna overtake Harden's standing in the team and the entire league at this stage in his career. I hope he rejuvenates his career here in Houston, but in all likelihood CP3 is going to be the 2nd best player on this team.