I take your point, but the complexity I’m talking about is what remains after you’ve done any readily applicable simplifications.

Any normal task has an inherent level of complexity (“add one” is a very simple task, so can be reduced down as you point out). “Invert a matrix” is somewhat less reducible. “Calculate a quaternion” even less so, etc.

We generally string dozens or hundreds of these simpler tasks into sequences to get what we want, and human understanding being what it is, there’s quite a difference in conceptual complexity between “fly the plane to [here]” and doing all that maths.

There’s probably a relationship to subitising here as well, the brain does seem to be able to process small groups of things, even if it knows those things represent larger concepts, better than larger groups of things. This is just speculation on my part though.