Let's look at that statement.
The max rpm your engine will turn on the ground gives you a needle setting that is going to be too lean in the air.
When the airplane is free to move forward, the engine has an easier time turning the prop. So the rpms in the air will be greater than on the ground.
When an engine turns easier, it turns faster. When it's turning faster, it actually needs more fuel going into it. Why? Because it's getting more air and you've not adjusted the needle to give it the extra fuel to match the extra air.
So you adjust the needle on the ground to give the engine more fuel that it's going to need in the air.

So your max rpm needle setting on the ground is going to be a too-lean setting in the air.