The company that makes the airplane is not normally the company that installs the interior anyways. The airline that buys the plane typically determines what the interior will look like (stuff like ratio of business to economy, overall # of seats, in flight entertainment, etc)

It being barebones from the factory is not a problem.

What does "usability" mean in the context of a prototype short haul passenger aircraft? It's got seats for people to sit in...

The competition in this market segment doesn't look like Middle Eastern airlines' first class cabin concept art, it looks like this: https://en.wikipedia.org/wiki/Saab_340#/media/File:Mesaba_Ai...

It’s a prototype. Surely easy access to the structure and equipment is a lot more valuable than nice seats and a finished-looking interior surface.