For one, there are objectively detrimental ways to organize code: tight coupling, lots of mutable shared state, etc. No matter who or what reads or writes the code, such code is more error-prone, and more brittle to handle.
Then, abstractions are tools to lower the cognitive load. Good abstractions reduce the total amount of code written, allow to reason about the code in terms of these abstractions, and do not leak in the area of their applicability. Say Sequence, or Future, or, well, function are examples of good abstractions. No matter what kind of cognitive process handles the code, it benefits from having to keep a smaller amount of context per task.
"Code structure does not matter, LLMs will handle it" sounds a bit like "Computer architectures don't matter, the Turing Machine is proved to be able to handle anything computable at all". No, these things matter if you care about resource consumption (aka cost) at the very least.