What's really being described there, I think, is the notion that you can serialize and unserialize some data and get, to some extent, the "same" data back.

Whilst that's more possible with protocol buffers or pickling (or whatever your language calls it), I can't think of any languages offhand which can round-trip any data. It's generally not possible to serialize objects denoting external resources - such as open file handles or network sockets. It's also often not possible to serialize closures, weak references (without dereferencing them), and not necessarily possible to serialize self-referencing objects: e.g. a list which contains itself - pickle can handle it, but I don't believe protocol buffers can do it.