Just to state why I believe it's Yolish and not from Scadrial.
Hoid has been around a long time. He was there at the shattering.
Preservation and Ruin created the human population on Scadrial, this means that it happened after the shattering.
Scadrial Feruchemy is genetic in the Terris people, the only ones who have it without hemalurgy.
Hoid is not therefore Terris, and I don't believe he spiked several keepers and became a version of an inquisitor.
Therefore I believe his feruchemy comes from Yolen.