I think it can be read either way. In some sections of narration the Culture is described as a true symbiosis, but elsewhere it's admitted and implied that the

I think it can be read either way. In some sections of narration the Culture is described as a true symbiosis, but elsewhere it's admitted and implied that the minds usually end up with more power than humans, and in multiple culture novels humans do get manipulated and used by drones/minds to various extents - e.g. Player of Games. It's not a symbiosis in the strongest practical sense. There isn't mutual reliance for survival: if all humans suddenly disappeared, the minds/drones and accompanying infrastructure would survive perfectly fine. But culturally (lower case) you could definitely argue that losing the humans would destroy the fundamental identity of the Culture. I like that while this topic was gradually touched on over the series, mostly the reader is left to make up their own mind.