It is intrinsic to the nature of the programming task that you cannot do it without having a large number of things in your head at once

That's true - but it does not mean that you can't give advices based on small parts of the code. Those advices would be helpfull or not - the author would decide eventually if they fit into the whole picture, but they can lead him into new ways of thinking. This schema can work since the cost of reading an advice is not that big.