Well mortal MA users still in fact exist.

Ashana Ikatu in Adversaries of the Righteous provides a model. She has Personal Motes: 10 and every Snake Charm, and the Martial Arts merit... but notably no special new merit that allows or gives her that Essence pool. This probably represents the same intent as the "Charms don't exist" mentality, the rules are there adjudicate things that happen in a fantasy reality, not be laws of physics that build it up like a computer sim.