If you have a system with "slow storage", caching is a way to optimize that to "storage that is sometimes fast". If you have a system with "slow storage" and "

If you have a system with "slow storage", caching is a way to optimize that to "storage that is sometimes fast". If you have a system with "slow storage" and "fast storage", caching is a way to abstract that away to just "storage". The author is arguing that the latter is the default way we should think about the concept of caching, which is a valid opinion to have.