You are still not getting it.

They are not talking about "unpacking on the fly for processing", but rather about "unpacking on memory to be able to call an opaque API outside your control that expects the unpacked representation". That requires copying in-memory to interface with that API.

Your approach only works if you are willing to "re-implement the world" to interface with whatever packed format suits your application.

With zero-copy serialization you don't have to do that.