One problem with orthogonal persistence is that the mechanisms it replaces (e.g. databases) are not purely for persistence but also for communication with other applications. But persistence is the only thing orthogonal persistence can do. If you don't examine how it is used, a database appears to be for persistence.

Don't look for the definition; look for the use.

You can't solve a problem unless you understand it. An important purpose of a prototype is to help understand the problem, not solve it.