The Obstacle In Forging
Lin Mu was not expecting that he would have really gained the affinity to the metal element, as it was not part of the True Gold Body Forging arts. It was not mentioned in any part of the technique, and it could not have been either.
Gaining an affinity to an element just from a technique was not something that was easily possible.
Most people either had elemental affinities right from the birth, or given to them by a physique or bloodline. This could be seen in the case of Little Shrubby, who had multiple elemental affinities by the virtue of his seven bloodlines, but even then the fire elemental affinity was the strongest.
Lin Mu had the earth elemental affinity innately and had been using it with the Mortal Strengthening scripture. That was the reason why the armor of the technique was so strong and quick to regenerate.
If it was someone without the earth elemental affinity practicing it, the effect would not have been as great.