The East Emperor's Heaven Stabilizing Art is a cultivation technique, not a Martial Technique.

The reason Luo Lie used it this way was to exploit the unique nature of this technique, which allows him to harness the essence of the sun and moon for his own use.

What is the essence of the sun and moon?

It is not just the power carried by the sun and moon themselves, but also the essence found in things like Sun Crystals, which is equivalent to the sun's essence.