The thing about reading code and understanding is logical reasoning, which you can do by knowing the semantic of each tokens. But the semantics are not universal. You have the Turing Machine, the lambda calculus, horn clauses, etc… Then there are more abstractions (and new semantics) built on top of those.

Writing code is very easy if you know the solution and the semantics of the coding platform. But knowing the solution is a difficult task, even in a business settings where the difficulty are more communication issues. Knowing the semantics of the coding platform is also a difficult one, because you’ll probably be using others’ code and you’ll face the same communication issue (lack of documentation, erroneous documentation, etc…)

So being good at programming does not really means knowing code. It’s more about knowing how to bypass communication barriers to get the knowledge you need.