Many people, especially in the tech field, have false perceptions about the inner workings of the human brain. We aren't rational automatons receiving exactly the data that has been sent out by the other automatons. That's for a multitude of reasons, the most obvious of that is the fact that only a very miniscule part of our thought is conscious (about 2% is the last number I've read about it). Even the fundamental inner workings of the brain differ from the idea you alluded to. Our brain isn't just a parser interpreting the data we receive - instead it is a black box constantly predicting what happens next, and only uses sensory input, both from the outside and from the inside of the body, to validate or falsify the prediction [1]. One of the obvious side effects if this is for example our tendency to ignore facts that don't fit to our current worldview.

So if I know that these things are as they are, and use them to communicate more successfully, is that manipulation? Then it would also be learning manipulation if kids are sent to school to learn how to write well, or how to do a presentation.

I had a situation with my kid a while ago. They were already tired, but had to take a shower. When I proposed that verbally, they denied. Then I showed them the warm water coming out of the showerhead, and they instantly agreed. So I got what I wanted (the kid getting clean), because I knew how to communicate successfully. But that isn't manipulation: I didn't lie, I didn't have a personal advantage at their cost etc. I just made it easier for them to anticipate what taking a shower would feel like.

So perhaps the distinction should be: If I can honestly and wholeheartedly argue to myself that my intentions are to the best of all participants, then that is communication. If I only care about my outcome, or even want to have adverserial outcome for the others, then that is manipulation.

But we can't use "not noticing" some mode of communication as part of the definition of manipulation simply because we all notice almost nothing consciously, compared to the sensory input we get every second of our lifes.

[1] A pretty approachable book about that, written from a researcher: How emotions are made, from Lisa Feldman Barret