The Dark One has many abilities that are not related to the True Power. Because the Dark One is an alien intelligence and god-like being that is not limited to mortal limitations that a human being is.

In fact, given that Slayer's powers are related to Dreamwalking, which is not dependent on channeling, I would say that the Dark One is more powerful than the True Power is. Taking this to another extreme, given that the Dark One exists in all parallel worlds and dimensions, and that there are some worlds where the One Power is not accessible, I would say this quite adequately indicates that the True Power is simply another tool to the Dark One. The Dark One, for all intents and purposes, is not a vast repository of random gifts - it seems that as the Pattern loses coherence, the Dark One gains influence over reality that has little to nothing to do with either Power.

Basically, I think you are treating the Dark One as the penultimate channeler when, to be fair, the True Power just seems to be a vague reflection of the Dark One's true powers and capabilities.