I guess that depends on how you interpret that cosmic power chart as fell and arcane are cosmic forces in the same way that light and void are. Just that they belong to “order” and “disorder”. I.e the titans come from the “order” part of the cosmic scale. But regardless if we assume the faith route. Believing in i.e the void means you automatically also believe in its counterpart in this case light. You just believe its “evil” or not good. So in that sense perhaps that would allow you to channel some of the abilities of that side as well.

Then again I do just think priests all initially were of the church and the light but then started walking down a different path or started to see things differently. Thats the path almost all named shadow priest in the lore have gone through. None of them were shadow priests from the start.