This is answered in the first paragraph of the article. Painting requires re-calculating the weight, strength and aerodynamics. Paint does not weigh zero, it changes the flexibility of the plastic, and the texture which changes flow.

But the article didn't give any ballpark numbers, so the interesting bit is missing, and we still know basically nothing.

It can very well be like the snake oil which makes you feel better maybe for the three seconds after you bought it. Or those gold plated audio jacks which are 0.0001% improvement in quality.