Eric's Heaven rank external techniques complemented his ability, simply because of its rank and him being a Heavenly Sage. However, for others, below the Heaven rank external techniques, Earth rank and it is still divided into minor grades, they need external techniques that are applicable to their abilities.
Similar to Eric's pyromancy and Celestial Grand Strikes, albeit, for others, their abilities were unconventional to the Murim World.
Most of the martial arts in the Murim World had their external techniques revolved around them. However, in this world, the abilities determined their future, so Eric had to adjust to them, tailoring internal and external techniques based on their ability, not the other way around.
Hence, it was unconventional, and it was why he called it "Unconventional Martial Arts."
