This theory is trying to prove that there is a third shard (other than Ruin and Preservation) on Scadrial.
Important Information:
Southern Scadrial has Metalic Arts, but they use them in a more mechanical way.
Feruchemy is the mix between the shards Ruin and Preservation.
Before the Lord Ruler's ascension feruchemy was genetically based and not mechanically based. Allomancy was extremely rare and genetically based and not mechanically based. Hemalurgy was used by some religions and not mechanically based.
The Lord Ruler split the population of Scadrial and made the Southern and Northern People. The Southern People now have "mechanical metallic arts".
How is This Possible?
Answer One: Preservation tweaked the system (possibly when he added the god metals)
This could be a possibility, but that would also mean that the Northern Peoples could learn to build something like what the Southern People have. Over the centuries that followed Sazed's ascension nobody has found out about this mechanical metallic art.
Answer Two: A Third Shard
I believe that there is another shard on Scadrial (when the shard came, I don't know). Feruchemy is not based on one shard, but on the mix of Ruin and Preservation. I think this is what the mechanical arts are. They are a mix of the shards Ruin, Preservation, and a third shard.
Summary:
At some point, probably after the Lord Ruler's ascension, came another shard that brought "mechanical" investure. Over the thousand years of the Lord Ruler's rain the shard blended its powers with the metallic arts, creating the Mechanical Metallic Arts.
The Shards Name: Innovation Shard