For this statement to be true and the only factor involved, we would have to be talking about copper vs. copper (which we are not) and the pressure of the copper against the barrel walls would need to be the same in both test samples (which it is not)

Traditionally copper is the main component of most jacketing materiel used in small arms, but it is not the only component. Sometimes brass is a major component, sometimes nickel, in some cases even steel. The alloy used varies from manufacturer to manufacturer and has evolved over the decades.

The thickness of the plating vs. jacketing also effects how easily the projectile can deform thus altering how much pressure is exerted outwardly against the barrel walls.

Projectile jackets are usually many dozen times thicker than the plating on projectiles available for hand loaders. Lead alloy (or other core materiel) is then poured into the already formed jacket.

The bulk of the plated projectiles used by hand loaders have soft swagged lead cores that are thinly plated