Is it Fair to say that a Preliminary user’s manual is a form of Prototype?

Technically, you could use the definition of the word "prototype" to describe a document, however - In everyday context, documents are not typically referred to as "prototypes".

You build a prototype of an item - like a car, or field generator, or a model of planetary movement - before you build it.

You write a draft of the user guide document for the prototype, before you publish it.

A "preliminary" document is a DRAFT not a protopype. A protoype is an model of something to show how it will look or work.