Its creators can 100% "change the code" though. That is called "training" in the context of LLMs and choosing which data to include in the training set is a vital part of the process. The system prompt is just postprocessing.
Now of course you and me can't change the training set, but that's because we're just users.
Yeah they can "change the code" like that, like someone can change the api code.
But the key point is that they're choosing to change the behavior without changing the code, because it's possible and presumably more efficient to do it that way, which is not possible to do with an api.
Now of course you and me can't change the training set, but that's because we're just users.