Roan was still the helper. That's why he offered to test it first. The solution, however, wasn't the one found in the Ruins of Yaz. Instead, it was the one revised by Kihin himself. Not only that, he and Roan improved it even more.
Roan took the solution, which immediately spread through his body, especially his meridians. Following that, Roan got rid of all the Divine Soul Power inside them. Of course, he also closed his dantian and stopped his Soul Power from moving.
Only after all of that did he allow the mixed Starlights to enter his body. Sure enough, the solution immediately took effect and acted on the Starlights. They made their way into Roan's own Dark Starlight while the solution acted. By the time the mix arrived at Roan's natural Dark Starlight, it had already lost more than 80% of its oppression power.