Many useful inventions seem blindingly obvious in hindsight.

Yes, in the end skills are just another way to manage prompts and avoid cluttering the context of a model, but they happen to be one that works really well.

It is not an invention if it is common sense and there is plenty of previous art. How would you otherwise dynamically extend the prompt? You will have some kind of function that based on the selected preferences add more prompt to the base prompt. That is basically what this is except that Anthropic added it as a built in tool.